So, when the DM rams these scenes down my throat, forcing me to play through them, despite my expressly stated wishes that I don't want to do this, what compromise is the DM showing?
Let's back up and establish something. Your original example was you 'surprised the DM' by summoning a mount. You then asserted that this was a clear signal that you didn't want to explore the desert or even play out crossing the desert. This is not 'expressly stated wishes'. This is implicit signalling. As I said, I would generally get the exact opposite signal from someone summoning an exotic mount to ride across the desert.
Now, if you stopped and said OOC to me, "I've got really no interest in playing out crossing the desert. Can we just skip to the Temple of Ix so we can retrieve the Golden Gizmo?", I'd probably be aghast. My first thoughts would be along the lines of, "How long has the player been unhappy with my game? I'm I really failing as a DM that badly? Why haven't I picked up on this players unhappiness before now? Why hasn't he discussed campaign direction with me privately if he has lost that much trust in me? Is everyone else this unhappy? Should we maybe just close the books and go watch a movie?" For one thing, this has never happened to me ever. I've had scenes that weren't playing out like I planned and decided on my own that I couldn't extend them the way I wanted to (usually because of under preperation on my part), but I've never had a player initiate that.
Well, I've been playing a few years now. It's generally not too difficult to tell when something is going to matter and when it's not. We're at point A. We need to be at point B. In between these two points are nothing that has anything to do with what we need to do at B. It's not really a big thing to realize that there really isn't a whole lot there.
How the heck do you know that? Have you been to point B before? Have you crossed the desert before? I mean, I could sympathize if the DM wanted to run the third or fourth time across the same desert in the same high granularity, but I generally avoid having retreads like that to avoid that very thing. Maybe it would be clearer if you would tell us what point B you were in such a hurry to reach actually was. But in general, I can think of tons of reasons why the desert crossing is important:
a) The desert contains valuable clues about point B. This could be all sorts of things like ruins built by the same civilization, which in turn have writings or frescos depicting elements of point B or tell you things about point B's history.
b) The desert contains widgets which will be useful when at point B.
c) The desert introduces monsters or themes which will later be expanded on when you reach B.
d) The desert introduces factions which control or occupy point B.
e) The desert contains allies which will help you in your mission in point B by providing clues, resources, or direct aid.
f) An opposing faction has prepared ambushes in the desert to prevent you from reaching point B.
Additionally, the desert even if it doesn't directly relate to the goal, the desert can still be important to overall play:
a) The desert is effectively the first level of the dungeon and is integral to the design of point B simply because it is just as much part of the layer of obstacles to be over come to complete your mission as the upper ruins or the first or second sublayer (in the case of a traditional dungeon) or the urban environment and the inner fortress (in case this dungeon is dressed in the trappings of civilization). Depending on my purposes, I could have even drawn the desert as a dungeon, with rooms, corridors, and encounters. To bypass the desert without interacting with it effectively lowers the challenge and makes the mission in point B less epic. Player disatisfaction with the design of the desert may hint that the entire design is flawed, since what the player is effectively saying is something like, "No dungeon crawling. I want to show up in the inner temple, kill the priest, and take the widget and be back in Haven before the end of the session."
b) The desert may have been invisioned as being an important section of some other theme than the immediate mission. There may be events in the desert that the designer envisions as being about the larger story of which the mission to point B is just a stage. Some earlier element of the campaign which seemed minor may here be enlarged upon or hinted at that it is going to increase in relevance. Think about the discovery of the underground library and the day of black sun in the Avatar series. It's not at all about the immediate mission of getting to BaSingSai nor is it even about the PC's immediate personal goal of refinding Appa, but its a critical story point. You might discover an abandoned Sun Temple in the desert where you learn from the frescos what the story is really about. Maybe the Sun diety is intended to personally show up and frame the larger mission. Things may occur here which frame the whole story. Even if that isn't on the table, there may be sidequests and events related to a particular characters back story. For example, one player may have always wanted a pet dinosaur. In the desert, there is oppurtunity for that. It's not important to the overall story, and its not important to you at all, but oppurtunity to obtain a dinosaur in a way that the player feels he's earned it, and that its been something of an achievement, may be a real highlight of the whole campaign for that player. The DM may have planned all sorts of natural character development to happen in the desert simply because it is down time from the mission which would be inappropriate when you reach Point B precisely because the mission is then immediate and pressing.
c) The desert is simply dangerous and therefore a doubtful proposition to cross it.
The desert could also be important at a meta level:
a) The DM may believe that the party is insufficiently well equipped or insufficiently leveled to take on whatever is at Point B. More time is needed to develop PC abilities and some largely superfluous encounters may be just the thing.
b) The DM may have prepared 50 pages of notes on the desert and is reluctant to abandon the work without at least trying to get players engaged in what he built. He's probably excited about it. He's put blood and sweat into it, and now you won't even try it?
c) The DM may only have some very loose and unfinished ideas about what is at point B, and he knows he won't be able to do it justice if you get there immediately. He probably can fake it for an hour or two, he may have a few rooms or encounters ready, but after that he's going to be just pulling stuff out of his butt and he knows that's when balance issues occur, that's when he ends up contridicting himself or forgetting important things. It's just not ready. He therefore needs to stall for time
Look, let me jump over the GM screen for a second and show what I would do as the DM in both situations. For me, the primary, single most important criteria is:
What is the goal?
What are the players and their characters doing right now and why are they doing it? That is the primary concern. If the goal was "hunting bandits in the desert" then fine, wander the sands. If the goal was, "Find the lost temple of Ix", then fine, let's go wandering. But the goal here is, "Let's get to Point B where we have to be in order to move the game forward."
So, with that in mind, when the players figure out a way to get to point B that bypasses the stuff I had planned on having them encounter because I presumed they were walking? Fine, no problem. Red Line ho! I might, possibly, use a skill challenge to narrate how the trip went - Each failed check is a loss of a healing surge. Arrival is guaranteed. Get there with no fails? You have smooth sailing. Three fails? You get there, badly sunburned, spitting lots of dust and you probably fell off a few times.

Total table time: 10 minutes.
So one of the big problems I percieve here is that you presume that the proposition, "I conjure a giant centipede" invalidates anything I have planned or allows them to bypass anything I had planned. In my case I know it wouldn't. Any time there is a journey, the first thing I figure out is what each possible mode of travel to the destination is like and what particular difficulties it involves. Riding a mount is always paragraph number two and in this case I consider it pretty much bog standard and expected. Now it could be that you surprised a novice DM and he wasn't ready, but I wasn't there so I'm not going to get into that. The point is that its going to take a lot more than a huge centipede to get across a desert in 10 minutes. Mass teleport would do it, but if you were a level that you could teleport, I'd be ready for that and it would have altered the focus I put on preparing for the session - less desert, more destination. Then again, I can think of probably a half dozen complications even for teleport.
In the Grell example, there is a very clear goal - revenge/payback on the grell. I might make them make a Streetwise check to see how long it takes to recruit six guards in a city, but, that's about it. Total table time: 10 minutes, 5 of which it takes me to write down stat blocks for the guards.
Without any context its difficult for me to judge the value of that approach. All I can say is that if that is the best resolution you can come up with, it makes the world you are building feel really empty and inorganic to me. And also, as a minor point to quibble with, if you use Streetwise or something equivalent in one of my games to do recruiting, you won't get 'guards', you'll get bandits or thugs. I've tried to explain why I can sympathize with the DM for wanting to play this out. I don't know whether this deserved to be played out. I would have 95% of the time, but I can also envision this being really screwed up by a DM who didn't really pour his imagination into the NPCs and try to make the interview process meaningful. As I said, most of the time I wouldn't have played out the shopping, and the few times I did I'd have very special reasons for it. Without the DM here to explain his reasoning though, I can't know what was going on here. But I can tell you one thing, I don't really feel any kinship to someone claiming the title of DM that doesn't understand why you'd choose to play scenes like this out.
It's baffling to me, this thread is. What have I really done here? All I've said to the DM is, "Hey, Mr. DM, this situation that you've put in front of us? Not really feeling the love. Can we just skip it and move on?" With the expectation that we would, in fact, move on.
Here is the thing. I've never had a player do this. I've never done this to a DM. What's baffling to me is that you seem to think this is normal.
Do you really hold this level of control and power trip over your players?
Power trip? Me doing the standard normal job of a DM to act as the provider of setting is a power trip?
That if they express a liking for a different sort of playstyle, you belittle and berate them and kick them out of the group? Seriously?
It's never happened. I've never had players do this. If it did happened, my first instinct would be to ask if everyone wanted to continue the campaign or if everyone was really this unhappy maybe we should do something else or maybe someone else should take over DM duties. Seriously, if the whole group was unhappy with me, I'd fold up my DM screens, put away my notes, and apologize for wasting everyone's time. If I can't keep the group entertained, I've failed as a DM and I just just hang up my hat. I don't know what else to say. This has never happened to me.
Now I have seen it done by a player to DM implicitly, and so maybe I'm just associating you with that particular player's behavior unfairly, but it's really hard not to see you in a negative light the way you've presented yourself here. The player wanted the DM to continue being the DM, he just wanted to dictate to the DM outcomes as well because he could not ever lose and he always had to be the center of attention and recieve validation of his coolness in every scene. I wouldn't have had the patience as a DM to sit through his crap, but one thing I did decide after that, no matter how much I disagreed with the DM I was never going to give him flak ever. If the DM is wrong on the rules, I'm not going to argue the point during a session. It's too damn hard to sit behind that screen and 'wear the hat' for me to ever sympathize with anyone who doesn't have absolute respect for the DM when sitting at his table. He may be a crap DM - I've 'stolen' tables from less than stellar DM's before, both by party concensus and by the DM passing the hat - but by golly I'm never going to be rude to anyone who takes on the burden. DMing well requires putting blood and sweat into the game.