Though I can barely play anything on guitar, whenever I hear a great riff in Rock Band, I'll go look up the tab and try to recreate it on my own.  Sometimes, though, I can't get the rhythm down right away and need to hear the song a few more times.  Playing along with the mp3 is fine, but once I know the riff, I'd like to play along to the song without the guitar track getting in the way.  Since Rock Band 2 will have a no-fail option, I'll finally have a way to play the other tracks to the song, and just strip out the guitar.  I'm looking forward to that.