There were so many other encores in the game they could've given boss battles or real band singers to. I have listed examples:
GB = Guitar Battle
RBS = Real Band Singer
Rock & Roll All Nite = Paul Stanley (GB)
Paint It, Black = Keith Richards (GB)
Even Flow = Eddie Vedder (RBS)
Pride and Joy - Stevie Ray Vaughan (GB)
You get the point. Who agrees with me?