"You still have a lose from not using tune and the instrument edges."

Yes if your bard is going to go up against one oponent which weapon you know, you might want to go weapon+instrument, but say you are defending your cabal against three characters and they might wield weapons you don't know, then I'd be a very happy man if I could use weapon+shield, cause that'd up my chances of surviving/singing requiem over three dead raiders corpses a whole lot.