You know, I was just thinking about this the other day.
I consider myself a decent DM. I'm not currently DM'ing, but I've mainly been a DM for the last 12 years or so. Sure, I played off and on when others wanted to DM, but I probably enjoy DM'ing more than playing, I think.
That said, I thought I did a fairly decent job, until I met my current DM. My current DM is doing a phenomenal job. He has this whole campaign planned out. Without going into detail, it's a very Tolkien-esque "save the world" kind of thing, which has taken us from 3rd to now 11th, and we're going to end it running the Rod of the Seven Parts module (great module, btw), so we'll end up around 15th or so. It's given me a new perspective on DM'ing. He's worked hard at integrating our own PC histories into the plot, and made it personal for each character, while at the same time maintaing a grandiose scheme.
There seems to be two main ways to DM - each has their own disadvantages and advantages.
Method one is the huge grandiose campaign style game. The main problem with this style is that it tends to railroad the PCs slightly. Like Frodo in LoTR, he doesn't really have any choice in what he does - he's got the ring, he knows what must be done. The good thing about this style of game is that it gives the entire game a cohesive feel and purpose.
The second style is the off-the-cuff style game. This is the style that I typically ran, and judging by his posts, probably the style that Angelsboi prefers. Basically, the players make PCs, the DM has a game in mind, runs it, and then comes up with something new next week. This style provides the most flexibility, and can literally have the DM asking the players "where do you want to go?" which I used to do a lot. Players are in complete control at the extreme in this style, and it can be challenging for a DM to come up with new modules, but the bonus thing is that they don't really have to tie into each other.
The *best* style is likely a mix between the two. An over-arching campaign that has a long-term goal, but allows for the PCs to get their at their own pace.
To specifically respond to your post, removing and subtracting players randomly doesn't work in *either* scenario if the party moves around all the time, unless the PC in question is some sort of high-level magic with teleport and discern location spells - probably not, I'm guessing. If the entire duration of the campaign is confined to a relatively small geographic area, then it would be more feasible, but that rarely happens.
Being able to anticipate what players do is impossible. Being able to wing it is a necessity. If you are wanting to run one-shot games involving the PCs, the best thing to do is work with the DM and the other PCs. Find out what their PC's goals and interests are, come up with ideas, and figure out how to work those into the DMs game. It won't do you any good to have an idea set in the ocean, and the DM moves the party into the desert. Understand where the DM is going so that you can anticipate your ideas.
Hope this helps; but it's probably just some more of my ramblings.
Good luck!