I think that this is a great idea for Harmonix. Get casual gamers playing a rhythm game with a standard controller to songs that they know and like. They can also buy into the thousands of available songs. When RB4 (or whatever) comes out, if they choose, they can give it a try. It will seem more familiar, and they'll be able to play all of those songs that they've gotten used to.
This will be a great way to get new players into the RB franchise while making more money for HMX. I'm excited for more songs... That's for sure.