The main issue is that you can do many of the things in Rock Band with bad technique. But if you use that bad technique in real life it could give you carpal tunnel syndrome.
Like I said, it's not that you learn nothing in RB. It's just you can learn a ton of stuff from a good teacher in a single lesson. Maybe I'm crazy, but I can spend hours with just a practice pad, some sheet music and a metronome. Just learning to do the basic rudiments with good speed, dynamics and timing is harder than anything in RB.
RB is fun and challenging. Just don't expect to learn basic drumming technique from it.







