The D&D family, Warhammer Fantasy Roleplay, Rolemaster, and The Riddle of Steel... four fantasy games. This seems less insane(or inane, maybe?) However, why do I need four fantasy games, and why would I need a fifth one?
*I'm a sucker for new game systems
*I'm a sucker for fantasy games
*I'm a sucker for anything designed in the style of "traditional/old school/whatever label you want to put on it when we all know very well what I'm talking about" style fantasy games
*It's twenty bucks (Although just for the basic book... I imagine a future release of Hackmaster Advanced will be at least another twenty, bringing the pricing in line with most modern core books.)
*I usually take away at least a little something, if not a lot of good ideas, from each game I acquaint myself with. I feel like being familiar with more systems makes me a better DM. It's always interesting to see how things are approached. Even if it turns out I don't like the game or I never run it, I feel that a new game book is always a worthwhile read.