Shield tanking question
 

I was just wondering if shield at was somehow better than normal ac? Just doesn't seem like the gain of ac would be that significant

Shield AC ignores the class-/level-based cap on item AC. (Not certain whether this is true for non-tank classes.) This tends to matter most for Paladin & Shadowknight in a tanking role, as Warriors have a higher item AC cap (assuming same/similar mechanics to this era on live) and they very much want the extra threat generation from their off-hand weapon.