Some songs don't need a guitar, bass, drums and vocals, which is why tracks produced for the upcoming Rock Band Network for (what else?) Rock Band won't have that as a requirement, either. Destructoid reports musicians will have no problem publishing songs that would only be playable for, say, vocalists only or exclusively guitar and bass -- it's up to the artist.
Sounds good to me. The more variety the better!
[image redit: flickr / dan [durango99]]