This is an interesting topic for me. One of the things you hear a lot about music games is that they get people to buy music through other avenues as well. For example, Aerosmith not only made a ton of $$ off of their game, but also saw their sales on itunes almost triple after the game came out. Why any band would be against these games is beyond me.
But this brings me to the question: how often have you purchased music because you played the song in RB and enjoyed it?
For me, I must have spent at least $200-$250 on itunes because of bands/songs that I liked in the game (I keep all my music on the computer so I can stream it to every room in my apartment). In my experience, I have probably purchased just as many songs from in-game bands that are not actually in the game as well. In short, the game has been a great addition to experiencing music overall.
Things that come to mind right away for me are buying a 40 song Roy Orbison anthology, about 30 Grateful Dead songs, a whole bunch of Bad Company, Journey, The Allman Brothers, and The James Gang. And I already had a TON of classic rock in my collection so that has been really cool to get turned on to some great stuff I hadn't thought of or had forgotten I liked (Allman Brothers).
It's amazing. 5-7 years ago people didn't buy music anymore. Now many people buy TWO different versions of the same song (itunes/other digital format and rhythm game).