Being a fanboy for the same reason racism is bad. It is an arbitrary determination you make that system X is better than system Y, and that leads inevitably to the assumption that a certain game is bad because it appeared on one system instead of the other.
Preferences are perfectly fine and natural. When I had both at PS2 and an Xbox, I favored the Xbox release of a game instead - and that was based entirely on the controller. Today I own an Xbox 360, and that is entirely a result of pricing and release dates. The handful of games that I would want to play on a PS3 that I can't get on my Xbox don't justify the cost of the system in my eyes, and the same holds true for the Wii (that sports maybe 3 titles I'd like to play).
Preferences are perfectly fine and natural. When I had both at PS2 and an Xbox, I favored the Xbox release of a game instead - and that was based entirely on the controller. Today I own an Xbox 360, and that is entirely a result of pricing and release dates. The handful of games that I would want to play on a PS3 that I can't get on my Xbox don't justify the cost of the system in my eyes, and the same holds true for the Wii (that sports maybe 3 titles I'd like to play).