Twohanded spam only works when teams are uncoordinated -- in sieges, for example, circumstances force the defenders to coordinate their archers to a degree, so nobody in their right mind goes on the attacker team without 1h + shield. Not to mention that the speed bonus you already get is enough to hit a 2h dude while he's swinging.
If anything, shields are currently overpowered because of how hitboxes work.