Anyone who is talking about "using real guitars" obviously hasn't played Guitar Hero.
It's not meant to be a substitute for playing a real guitar. You are comparing apples to oranges. Just like how DDR won't teach you how to dance.
Now what it might do is give you a bit more rhythm and prepare you for a little hand movement here and there. But of course it won't make you a guitarist.
Plus, I don't know if this somehow escaped you, but real guitars cost much more than a game controller.
I also challenge anyone who actually plays a guitar to sit down with Guitar Hero and play the expert songs, especially "Ready to Rokk," "Bark at the Moon," and "Cowboys From Hell." Sure, you probably stand a good chance and picking it up, but the skill sets really aren't the same.