I do everything except drums in Rock Band/RB2. Well, I do drums occasionally but I'm really bad - that is, I can't do it on expert so I don't consider it worth the effort. And yes, I realise that sounds horribly elitist. Also, I live in a tiny room at university so it's not really practical to set up the drum kit even if I wanted to play.
Also, in RB1, I did the entire Band World Tour (including Endless Setlist, getting 260/290 stars) doing expert guitar and vocals at the same time. I'm not really bothering with that sorta thing now, I don't have room for my makeshift mic stand (a guitar stand with the top bit reversed) in my uni room so it's a bit hard to arrange. I'll probably have to do it at some point though, if I decide that I want the 'Big In...' achievements.
I mainly stick with guitar and bass in GH:WT, I'm not a fan of their vocal engine but I do it occasionally if there's a song I particularly feel like singing.