Well, once he takes off somebodies arm, he can then use the arm to batter down the cockpit door, knock the gun out of the pilots hands and take control of the aircraft. Imagine what he could do with 3 arms. I'm sure everybody has seen the Nationwide insurance commercial where the guy does the hand shadows with 3 arms/hands. I can't think of anything more frightening.

But have no fear, an unarmed tsa employee with no or little hand to hand combat training is on the case. Meanwhile, if the police were not actually needed, there apparently was no danger anyways.