Yes I buy books even when I'm absolutely sure I'm not going to use them in an actual game. I did for years before I found a regular group, and still do.
Part of it is that I look at "playing" (by which I mean being knowledgable in the mechanical, and setting, underpinnings of a game system) as being a skill; it's something I want to develop and be able to do more effectively, and that means absorbing new material that comes out so that I have expanded knowledge of a subject and greater material to work with.
That said, I did regard developing said skill as having a practical context - I presumed that at some point in the future I'd find a group where I could put all of these resources into play. Well, I found a group and...they didn't really care that much that I knew of hundreds of different options across as many third-party books. They were not only happy with the basic rules and a few first-party supplements, but viewed third-party offerings with deep suspicion (about how "balanced" they were).
Worse, even when it was my turn to GM, I found that knowing all these different rules and actually devoting the time and energy to building characters and scenarios that used them were two very different things. Circumstances in my life were such (and still are) that I can't really run a campaign unless it's a published one.
Both of these limitations meant that I underwent
supplement burnout pretty hard, and it's taken my RPG buying down a notch in the last few years (also, finding a great
class-less character builder for my system of choice - 3.5/Pathfinder - which eliminated the need for a lot of supplements, didn't help).
I still buy a lot of books, but at this point they're much more focused on either what I can use in my current game, or are old and out-of-print materials that I want for a combination of nostalgia and collector's enjoyment.