"Do what you need!" is the single most useless piece of advice I ever received as a DM. It seems obvious to me that DMs should always do what they need. It's in the job description, practically. "DM's control the game." Ergo, they can do what they want or need to make the game go.
What do I need to do? What is cool about this monster? What encounters and plotlines can I use it in? What does it do when adventurers aren't showing up to ruin it's day? How do I react to what the PC's do to it?
I don't know what I need it to do before I use it in a game. What I need at that moment I DO use it is for the game to tell me what it does. I don't need reassurance that I can do whatever I want. I need empowerment.
You can at least start with giving me dungeons and dragons in a game called Dungeons & Dragons.
The thing is, when you start doing things like detailing an orc warcamp, or a dragon lair, you also detail things like, say, the animals that live nearby, the traps that they use, the leaders that they have, the variants that occur, and you have something that any DM, new or old, can plunk down in front of her party tonight and use, without any major prep required.
And each individual bit can still be broken out and tossed into a compendium for those who want to prepare their own dragon lair.
Major difference being, with enough context, I no longer have to do it all myself. I export the boring work of detailing an orc camp out to the Monster Manual, and I can spend more time worrying about where I'd put that orc camp in my world.
FWIW, some 2e monsters, and many 1e monsters (especially humanoids) had this kind of built into their monster descriptions. Check out the orc description in the 2e MM: aside from a map, it gives me all I need to plunk orcs down anywhere in my world. An orc statblock is a fairly boring thing in and of itself (looks like most other humanoid statblocks). An orc camp is a useful thing, though.
I also think the "quantity over quality" approach to MM's is distinctly unnecessary from a publishing standpoint. After just a few books, the market reaches a saturation point where moar monsters doesn't add much to the game. Statistically, if you use 5 different statblocks in every combat for 10 combats per level over 10 levels (roughly the amount of time people spend playing a campaign according to Mearls), you use only 500 unique stat blocks (with even distribution over those 10 levels no less). That's less than two monster manuals, let alone tricks like reskinning, modding, templating, and ability swapping.
Thirdly, if you provide DMs with a way to usefully generate their own statblocks (a very useful addition from 4e), you don't NEED to give them squat. You will have all the statblocks you ever need right there in front of you. If you run out or feel like changing things up or want a particular thing, you can make it.
It's not my job as a DM to sit alone in my bedroom and carefully hand-craft an entire night's adventure from a few statblocks and my own chutzpah.
It's my job as a DM to run an active D&D game, and that means filling in the blanks for me, or at least giving me something like MadLibs so I can fill them in on the spot with a random die roll or something.
DMing, to me, is about playing the game, not preparing to play the game.
Although I disagreed about being able to use a Dragon/Solo as a Villian, I'm going to agree with most of what is here, especially the last statement.
From what I can tell, 5E is moving toward 2E as far as flavor, fluff, ecology, etc. Even the Essentials MV and MV:TttNV have incorporated much more fluff and flavor to their entries. I think WotC is on the right track and will continue this path.
In 2012, I think it would make perfect sense to release a MM with fluff as a Book/PDF and an MC with only crunch as a digital tool only.
Like you, I like to PLAY the game not prep the game. I DM sandboxy, so having generic information that I can either use immediately, or quickly reskin is essential for my games.