The experience is a bit weird anyway, charting piano and other instruments to guitar is hardly "realistic" is it? But that doesn't worry me either, it's fine by me.
And one guitar that plays whatever notes from John and Paul's guitars makes for the best gameplay at that point in the song? That's not remotely realistic either, but does it detract from the gameplay? No, not for me anyway! I'd much rather have that technique than being forced to be either George or John all the way through an entire play session, which would be the "realistic" alternative.