I've always looked on shield skill as to how good you're at using your shield to full advantage. With the increasing 'forcefield' with higher shield skill I look at it as that the wielder gets better at moving his shield around to protect from melee strike and arrows.
This is a medieval game. Everything should at least be partially real, otherwise there is no point. I don't fire arrows to fire at invisible walls. I fire them there to hit the target. You cannot tell me that a man can be in battle AND block an incoming arrow at once. If he can block an arrow with his tiny shield, I may as well be able to block them with full metal gauntlets. Hell, Mythbusters even has an episode on this. It is impossible.
I copied that from the above post. It is extremely hard to time these things whilst fighting, and guarding yourself against an object that is going 90+ miles an hour that is the same size as a quarter.
Also, most people are probably trolls but IDGAF. Point remains.