You can use a single world as per your preference, but the key to making it all work is that you run 1 shots (single short adventure provides at least 1 level not more than 2). Personally I choose to take the route of having fun with the idea that I don't have to have a persistent environment. I already have one well developed world that I run (it's in my sig as well) but this place gives me the unique opportunity to create worlds that wouldn't fit properly into my own...
For example my first adventure I'm writing up takes low level players to a psychic fortress in the astral plane. I never get to do stuff like that in my low fantasy world (well at least not until they are epic level equivalent).
There are several DMs going your route though, so that's more than fine.
Also, thanks for the doctor who reference, I didn't make that connection before, but it makes sense.