The point of the game is the instruments
It's a rhythm game, it doesn't have to be played with the instruments. Guitar Hero III gives me the choice to use the controller, Rock Band does not.
Rock Band would cost me £39.99 just for the game. The instruments all together are £99.99.
Then, Guitar Hero IV comes out with it's incompatible drum kit and I've got two sets of plastic drums taking up space in my house.