I take your point that it is only a game, but surely the satisfaction gained from being able to properly play all the songs on guitar hero is more worthwhile over getting a top score for pressing buttons?
I'll say it again, some people just don't WANT to play the guiter, just because they want to play Guitar Hero and have fun doesn't mean they are quitters or lazy, in fact I think people who assume that are arogant, some people just want to press buttons and have a good time.
If I chose to play the guitar, fair enough, this means I want to play the guitar, if someone decides "No, sorry, not interested in playing instruments" and instead just wants a game they can stick on for an hour every now and then, this doesn't make their choice any less reasonable.
Saying that, the people who play Guitar Hero and make a massive deal out of being good at it I think are a bit strange, it is at the end of the day a game about pressing 5 buttons.