I have so much White Wolf stuff I've never used it's not even funny. Most of the Mage: the Ascension and Wraith: the Oblivion books, most of the Werewolf: the Apocalypse books (I played one game, but that was before I had any of the books). Most of the Vampire: Dark Age books (again, we played a campaign set in the dark ages, but that was before the Dark Age book was even announced). Many of the Clan/Tribe/House books are very cool and breath new life into groups that I wouldn't have originally thought cool (the first Clanbooks Nosferatu and Lasombra were amazing! The second ones for Vampire Revised, not so much, but I bought them anyway.).
I *love* reading Aberrant, Trinity and Adventure! books, despite limited chances to ever play any of them.
If I want to read a book, I buy it. Doesn't matter if I ever 'use' it. I don't 'use' comic books or novels, either. If it's a good read, it's a good read, and some setting books, such as the Scarred Lands books, are often good reads. Eberron books are a bit dry, I'm not sure why. Realms books are often interesting.
I have no idea what Greyhawk books would be like, since they haven't had one since the Living Greyhawk RPG doc... But I'd buy them, just because I'm like that.
D&D splatbooks, on the other hand, have to have something in them I would like. I don't have any of the 'Races of' books (except Races of Eberron, which was a huge letdown). I do have all of the Completes, the PHB2, etc. which I *do* use.
The only reason I have a job is to pay for my reading habit...