I play guitar as well, and this summer i plan to rock out on GH almost everyday. The game and the real thing are so different, i don't really see how you can be offended by it in any way. It's fun is all, it's not like anyone claims that playing GH makes you a real guitarist.
I play hockey as well, does that mean i should avoid all NHL video games? And what about Madden, should football players avoid those? And should low life thugs avoid GTA, as it dumbs down the art?
Anyways... GH3 will be worth getting just for "Knights of Cydonia". That may be my favorite rock song of all time, actually.