I use a shield when main tanking or going against a group with a caster in it. Shield bash is key in taking down groups with casters in it for me. It provides: 1. Extra Ac 2. An attack with a stun component to stop casters 3. The stun can also all you a precious few moment to recast one of your keybuffs while not getting hit or give your healer a couple seconds to get another heal off.