Can't we make one of those sticky?
The answer stays the same: Greyhawk, the way it's used now: hardly. Enough to base homeworlds on if you're not experienced enough to create something from scratch, but not too much, which would pen you in. GH has been doing this job for quite some time now, and can continue doing it.
And it's doing a great job: It's vanilla enough to act as a base, there's hardly any people who really hate it.
FR might work, but it might chase away those who hate the Realms (yeah, I know, a lot of people hate the Realms because they are prejudiced against FR-Fans, pathetic as that might be, and others just don't know how to use the Realms properly).
Eberron can't work because there's too much weird stuff in there. The basic setting should not have weird stuff in it. That's for other settings. If the base setting is already weird, it would skew the perspective. You'd either force all other settings to use that stuff, or you'd basically have a setting that's "weird and different" because it's not. It's just not using all that crazy stuff that has been declared norm. Plus, Eberron really isn't for everyone.
Designing a brand new setting just for the core rules seems way too much work. They can always design a brand new setting and sell it as a Campaign Setting, but they wouldn't do that if it didn't offer something new, and that means, again, that it's not really suitable as base setting.
Ressurecting an old setting is not a good idea, either: It's unnecessary, many of them are too weird, or are just another vanilla fantasy setting. Why fix what isn't broken? Stick to GH.
OGL settings are right out. They'd probably have to buy those, and that would drive up the game's price - while not improving the game in any significant way.
No setting at all is partly the way. There should be something do base everything on, some defaults to use when you don't want to bother with that sort of thing (like creating your own pantheon when you really don't care what the gods' names are). It should be subtle, but I think the way it is now works fine.