I'm sympathetic to the 4e setting designers' concerns here. I get that if you put setting material in the Core Books and people start to associate that with playing D&D, that it then becomes something that you kind of WANT to include in every product. Like, if people are used to the Feywild, or the Elemental Chaos, or teleporting blink elf PCs, because that's the "default," the thrust is to include the "default" in every setting so that someone coming from just the Core Books still gets to play with their favorite game element. Arguably, this is why so many settings have elves and dwarves and halflings and gnomes, and why they're kind of vaguely similar to each other in most settings: the default intrudes on other settings by its nature as a default.
I kind of get that pressure. Which is part of why I've come to the idea of "no default, just examples." That way it's clear from the outset that D&D itself doesn't necessarily involve X, Y, or Z, but it is only by the choice of the DM and the group that their particular game has it. Which frees you up when you're making a setting to do what makes sense for the setting without having to accommodate an assumption that X, Y, or Z, because it's in the core rules, should be part of every D&D game.
