I think that you'd do better to double the AC bonusses that shields give instead of using cover mechanics. It's much simpler that way.
So: Buckler still gives +1 AC
Small Shield=+2 AC
Large Shield=+4 AC
That makes shields an incredible advantage when combined with armor. It's even feasible to use a shield and no armor--you've still got the same AC as a fighter in scale mail. However, this method doesn't introduce directional mechanics or AoO negation to the combat mix.
As to the historical discussion, I believe that you're in the wrong. As full plate armor was developed, shields fell out of favor (they were no longer necessary as full plate provided sufficient protection--this is especially noticable in late tournament armors--where protection was most essential and one could afford to give up flexibility--which eschewed the use of shields). At the same time, weapons like pikes, bastard swords, greatswords, military picks, poleaxes, and maces began to replace one handed swords as the primary knightly weapons of war. (One handed swords no longer had the power to reliably penetrate fullplate).
As the penetrating power of firearms and crossbows improved, armor grew even heavier, and the less essential parts (greaves, sabatons, etc)were jettisoned so that it didn't grow too heavy to wear (that's the origin of half-plate). As firearms continued to improve, breastplates became even heavier until the only parts that remained were a leather coat, breastplate, helmet and guantlets. (Elizabethan/James I/English civil war era). Around this point, the shield came back into favor for a brief time when the spanish infantry (longswords, breastplates, and small shields) defeated French pikemen in Italy. It didn't last though. Later armies abandoned shields in favor of muskets (infantry) and pistols and sabres (cavalry).
Samuraii are another story entirely. My understanding is that they belonged to a much more tradition bound culture than medieval europe and were expected to use bows, naginatas, and katanas in battle. Seeing as two out of three of those are two handed weapons (and the katana was usually used in two hands) there's not a lot of room for a shield. I doubt that they allowed their tradition to dictate an inefficient form of combat to them though. More likely, their tradition crystallized an effective form of combat and held it in place for all sides. My understanding is that their armor was sufficient even without a shield.
Psionicist said:
Point given, however I still belive my own system is much better. Not right now, but after a bit details it's all there. The keypoint in my thread was not exactly the two weapon fighting part, but the shield part. Imagine you are fighting an orc yourself. Would you rather fight without shield where ALL direct hits will hit your body, or with shield where you can easly dodge attacks with the shield. Personally i do think the "warriors with greatswords are fearsome brutes capable of dealing out huge amounts of damage" ARE fools. I have read some books on history, and unless you are extremely dexterous you cannot win a fight without a shield. The actual armor doesn't really matter, as long as you have a shield.