The idea of shield is to sacrifice some offensive capabilites for more defense/utility.
How about for shield to have some armor regen (up to point)? Let's say equipping shield gives you 2X armor. Each combat turn X armor is regenerated, up to 2X (provided by shield). In current state bonus (aka "2X") is too small to consider even with suggested regen, so values should be adjusted.
Combined with suggested above shield knockdown as attack of opportunity talent, and we have shields as viable choice.
