I agree with you. The thing is, you won't hear the disc spinning, but you will still have to have it in there. Also it is pointless because it slows down you 360, and if you ask me, i would rather hear something that makes a tiny noise, rather than playing on a laggy or slowed down 360.
The only reason why you pop the disc in is to verify that you bought it. The actual reason to install it on the hard drive is that you won't have to use up its laser which will lengthen its life and the load times will be much faster