1.Common Sense, I don't think he HAS to just hit them up and down.
2.In Armaggedon Dan Fordan disguised as Shang disguised as Ermac threw Quan off the thing. Since Shang gets so into his identity theft (i.e. Sonya's things) Evidense that Ermac could do the same.
oh, and it's just as fast to move your arm as it is your hand.
not neccerily, your hand can definatley move faster, your arm cannot move as quick as a hand...also its more cumbersome with an arm, all kain has to do is shake his wrist, this dude has to move his arm and quite eraticly too it seems to make an effect, kain lazily moves his hand without too much effort, can Darth vader style choke, throw beings about etc etc