I think it absolutely does need a default setting but not in the expected manner. Somewhere upthread someone suggested adding a 4th core book to the traditional trio of PH, DMG, MM. Call it the Campaign Book (CB.) This is a brilliant idea.
I play 4th Ed and I enjoyed it but it's approach to world-building was COMPLETELY 100% Bass-frakking-ackwards. They created all the parallel planes (ups/ downs/ sideways) and god/creatures what-have-you's. Real, CREATION-LEVEL stuff that binds the whole fluff together and then, almost as an aside told us that the actual playing surface of the game was essentially up to us to come up with or at best was plug-and-play. Just snap whatever old setting you want in the middle of this great cosmological donut we've left you. Too bad everything else about the game is built around you using our assumptions about how the whole shebang fits together.
This is much worse than giving us no setting at all or just the bare bones of the Nentir Vale. Either of those I could work with.
That off my chest, my suggestion would be to keep the core rules as fluff light as possible without sacrificing any important aspects of the mechanics. Just keep it vague. Even Cleric Domains can be generalized as God of War, Grain, Furniture, whatever. I can name them if and when needed. Wizard spells? Fireball & Magic Missile work just fine. Themes and Backgrounds? Blacksmith, soldier, scholar are acceptable, right?
Then, SIMULTANEOUSLY, with the publication of the Three, put out your first (of many!) CB's. Gods, Devils, History, Cosmology, Setting specific Themes, Traits, & Backgrounds, Unique Creatures, Named Spells, Magic items. Get crazy. And then get more crazy and actually SUPPORT these settings, Now, THAT's some OLDE SCHOOL Product Support right there.
Soon(ish) after release a Campaign Builder's Guide for DM's that like to homebrew to their hearts content. Full of rules and advice on tweaking the Core to create all kinds of D&D-esque settings and adventure types.
Settings ARE important. At least equal to mechanics. In literature we learn that setting defines themes. Westerns are good for telling certain types of stories better than Detective, War, or Horror stories are. Game of Thrones isn't the kind of world ideal for Dungeon Crawling. Don't make it have to support those kinds of adventures if you can help it.
Just my thoughts. As always, your milage may vary.
Dave