You need an extra guitar for your Rock Band so you can have a four person band with a bass player. (That's so your friend who can only play on easy doesn't feel bad about being left out)
We own Rock Band here at This Next... we don't ever actually have time to play it, but it's here and made a great prop in our "The Singing Office" Audition tape. One ThisNexter (who shall remain nameless for the sake of her family) did a wicked guitar solo and ended up jamming out on the floor. We didn't have the heart to tell her that we forgot to plug it in...