Majoru Oakheart
Adventurer
Not really. I don't like coming up with worlds either. Which is why almost all my campaigns take place in Greyhawk or FR. I find most of my games don't really focus on the flavour of the world itself and work in just about any world with generic fantasy sensibilities. If I am running the Tomb of Horrors and the PCs spend most of the campaign in the dungeon then the politics and layout of the world are nearly inconsequential to my campaign. But at least if I set them in a prewritten world that extra information is there in case it happens to be required(for instance, the PCs insist on going to a nearby town for rest that I can at least add a touch of flavour to the game).In this regard, are the planes/cosmology different in some way from "that kingdom over the mountains"?
For me, I'd say yes. Back when I was much younger and we played 2e D&D, we used to sit around and have metaphysical discussions about D&D. About the nature of the planes, about why they were created the way they were, why the denizens acted the way they did. We'd discuss what would happen if Elminster went through a portal and ended up in Dragonlance and met Raistlin. We'd discuss what would happen if one of the Wizards from one of our campaigns ended up stumbling across the Demi-plane where a Wizard from one of our other campaigns lived.I certainly wouldn't take that as any measure or evidence for a lack of creativity! I think just about everybody in this hobby takes inspiration from all sorts of sources, and not just fantasy and the rulebooks, either. However, even if you'd never come up with Mechanus on your own, does that mean that its existence, properties, and relationships with the rest of cosmos is needed as a default assumption that the rules are built around?
To us, D&D was as much about the meta-universe as it was about any particular D&D game. It was always assumed by us that every D&D game was connected to every other D&D game since they all took place in the same cosmology. They were all different Prime Material Planes...but they were all tied together by the same ethereal place, the same astral plane, etc. It made the game feel...well, bigger than any one particular game. We loved that part of the game possibly more than actually playing D&D itself.
But I love meta-plot. We'd spend just as much time discussing Rifts and what if scenarios for that game. We almost never actually played it because the game itself fell apart too easily, but we were interested in the setting and characters. To me, the planes, Orcus, the lords of hell, Sigil, demons, the Blood War, and so on ARE the setting and characters of D&D. Each DM's world is just a layer thrown on top of that.
However, my main point was that without it, I'd likely invent something stupid like a Plane of Pizza where souls go to die and turn into gooey mozzarella. I don't want to subject my players to that, so I'd be happy to use someone else's better ideas.