What am I missing here?

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I'll admit to being a fan of both The Beatles and of Conan O'Brien, but I think there's something amiss in this clip.  Conan Plays Rockband

The problem I have starts when they start playing the game.  By my count there are four people on stage... each has some instrument in their hands.  But I only see 3 tracks of people playing.  Conan does make a comment about one of the guys is there to help with vocals, but if that's all he's doing why does he need anything in his hands but the microphone?

There's some giggling at some point when its clear that Conan has no idea what he's doing, but is that really that much worse than a guy who is playing but isn't connected?

What am I missing here?

Mac