The common gripe about Rock Band is that you're not really making music. It's just a game! Big whoop!! Well, let Harmonix boss Alex Rigopulos address that issue:
First of all, in Rock Band the singing gameplay is really singing, and the game feedback on the singer's pitch really does help people become better singers. And while the drumming gameplay isn't an exact replication of real drumming, it's certainly a sufficiently close simulation that players develop skills which they can directly apply to real drumming. I am continually seeing people migrate from Rock Band onto real drums naturally.
And the guitar? Well, that's "a far more abstracted version of the real thing," Rigopulos says. It does teach players about rhythm, or rather, give them a sense of rhythm, but it doesn't mean an expert Rock Band guitarist can shred a real guitar.
According to Rigopulos, "We don't see Rock Band as an educational tool for guitar so much as an inspirational tool."