yeah, i can't really disagree with you in principle - but other than hammering skins in a rather eclectic fashion, my musical skills are limited, so something like this is great. i think all that really matters is how much fun it is, and despite the time wasted on guitar hero when i could be more productively learning guitar, it wouldn't be as much fun. the end result may be more satisfying, but when i'm having computer game time, i'm not trying to better myself, i'm going for escapism - and for many people who enjoy music, but suck at it, this is escapism into a world that they normally would never have access to, but always wished they did.
anyway, we all know it's a waste of time. isn't that what computer games are? i'm not disagreeing with you - if you were in a room full of people who could actually play instruments and sing, it might be a bit weird to play a game that is mearly mimicking those talents. although saying this, the two biggest GH freaks i know are accomplished guitar players - so there is obviously more appeal in there somewhere than pure immitation.