Maul is perfect for the movie. He's menacing. And he's a perfect jump off point for the introduction of the Sith: he is pure uncontrolled rage. Regardless of the false fronts put up by the other Sith (controlled, aloof, omniscient, refined) when you come right down to it they are alll at their Core what Maul is on the surface: uncontrollable rage. it's the source of their strength and their greatest weakness.