In the comments our Rock Band Network preview, Destructoid reader Xzyliac had an interesting question.
"Do you know if Harmonix is requiring four tracks for every song?" he asked. "I already know the guitar tracks are 'flexible' but what about if, say, I'm a folk singer-songwriter and all I've got is guitar and vocals?"
While the question was presumably answered by a screenshot which only displayed a bass and guitar part, we checked in to make sure our eyes weren't deceiving us.
"Yes, you can author songs with less than four instruments," an MTV Games rep told us. "You can create songs using any of combination of the instrument tracks, between one and four."
So not only is guitar/vocals folk rock viable, but the door looks pretty much open for other styles of music -- hip-hop songs with only strong drums, bass, and vocals, for example. A classical, acoustic guitar piece, or something for solo voice.
Of course, you'd have to wonder how much fun a track like that would be to play in Rock Band. Should the price of a track cost less because fewer parts had to be authored for it? (Artists can set their own prices per song, from $1 to $3.) What do you think of tracks with "missing" parts, and how much would you pay?