Shield bashing isn't that difficult to balance, just make it real - a short ranged blunt strike that does little damage to armored men, is a bit slow to recover from. Used like a normal strike, you hold attack while holding block and you bring your shield closer to your body, making it possible for arrows and spears (generally speaking) to hit you through it. Then you release the right mouse( block) and swing your shield, hitting poorly damaging and possibly knocking an opponent down or at least pushing him back. Think of it as of a 10-15 dmg sledge hammer- "can crush through blocks" "unbalanced". Thus it would be risky to just hold down the bash because you may get hit by arrows penetrating, and the recovery time will make it useless as primary attack.