I have no problems with a default world to be honest. No, it doesn't have to take up pages of text - two, three pages was enough to give me Mystara after all and that's including a full page sized map. A general, fairly generic fantasy world where you can say, "Ok, here's the bare bones of a world you can use as a template to create your own worlds." is invaluable in core rules, which do have to accommodate that new player as well.
I've never really understood the negative reactions we get from having flavour in the core books. I mean, 1e had the Racial Relations table. It flat out told you that race X and race Y don't like each other. Yet, that's apparently okay, but, a paragraph saying where Tieflings come from is too intrusive? Really?
Every bit of flavour you include in the game will go some distance towards creating a core world anyway. You might as well start with one and then at least you have some level of coherence.