Why? There are dozens and dozens RPGs out there few gamers can give each one a chance. Fantasy games seem to be the most numerous and with established games like the editions of D&D and Pathfinder that people enjoy gaming with each week why should they stop spend money on something else and devote time away from their campaign to it in the hopes they like it?
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I think it is worth recomemnding for people looking for a very specific niche. It is rules light (in the good sense) and fairly easy on DM preparation. There are alternative (good!) systems out there with Pathfinder and 4th edition Dungeons and Dragons being the most prominent. I currently referee Pathfinder so I am hardly adverse to using that system.
What I liked about Castles and Crusades was that it managed complexity fairly well. It doesn't feel like a perfect system but, in fact, some of it's more glaring imperfections tend to be charming (like in the AD&D sense). It is a system built by hobbyists, so far as I can tell (or professionals who are really good at mimicing that feel).
What I liked about 4th edition Dungeoons and Dragons is that it is really easy for a Dungeon Master. Encounters are easy to build, easy to plan and the combat system means that no encounter will be over in a heartbeat. That can be a huge preparation advantage for the DM.
What I liked about Pathfinder was amazing production quality and a thoughtful revision of a character centered game. In no other modern RPG is lovingly crafting a character jsut so much darn fun. Tons of options and many ways to get to the same outcome.
So I think there is a lot of room to like many systems, each for their own excellent merits.