Well, maybe this'll turn into some sort of cross-blog dialog, but Blacksteel followed up my response to his post on in-print and out-of-print games here. He notes several games that he really likes that didn't make it, as well as some guidelines for what makes the cut when it comes to taking up precious space on his gaming shelf.
I'll admit to being more of a collector than Blacksteel is, although having to move all my possessions three years ago has certainly blunted it. I gave away over 100 RPG books to the local public library as part of their youth program (some youth somewhere in Ohio might currently be playing the Hercules and Xena RPG because of me). Nowadays the internal process for "should I buy or should I not buy this RPG" tends to go like this:
1. Am I going to run this game?
Yes: buy it.
Maybe: go to 2
No: go to 2
2. Is it something that look really interesting, like maybe I could steal something for another game?
Yes: go to 3
No: don't buy it.
3. Is it cheap?
Yes: Wait at least 24 hours, considering question 2. If still yes, then buy it.
No: don't buy it.
So, for example, the Iron Kingdoms RPG is a game that I could run, although it's unlikely. It is, however, very expensive, so I took a pass. The copy of Interface Zero I found in a clearance section of my FLGS, however, is something I might run, and it was interesting looking, and it was cheap. Thus, a purchase. Now I will in all likelihood cull that book some time because I am starting to feel the pinch of space, and unlike Cyberpunk 2020, Interface Zero isn't exactly a classic. Actually I've likely got a big cull coming up, just so I can free up some space and clean up the man cave, but I'll save that for another day.
What's interesting for me about Blacksteel's article is the issue of pondering whether to buy additional sourcebooks. Aside from D&D (and the ersatz Pathfinder) I'm not aware of too many RPG's that now mandate additional books in order to play. The GM's book for Mutants & Masterminds, third edition had lots of helpful information, but nothing critical. Frankly I wouldn't mind seeing a few more RPG's that had a "player's handbook" that just had info the player's needed and a expanded book for the GM. Instead of having one book that is passed around, you'd have the one book, plus the addition demi-books for the players. I'm probably wrong about this because it isn't the primary business model of the industry, but I still think it is a good idea.