The gameplay is admittedly growing a bit stale. And, yeah, they're making too many new versions. Either it's just one-upmanship, or licensing issues. Who knows?
The games are just so dang expensive, and they're so easy to make. I bought Guitar Hero 3 and loved it, within about a year another guitar hero game comes out. I decided not to get it. No later than that they expect your to pay $200 for a single video game. By then, I decided that it was ridiculous.
RB3 is adding Pro mode which helps you get started in learning real instrument skills. However, it costs another $200 for the Pro controller (There are pro modes for 4 instruments, and 3 of them require new controllers). But a 50 dollar disc for 80 new songs is a good deal since Rock Band/Guitar Hero songs cost $2 each.