It is perfect in its current slot. In shoe would mean their is a a -100% max hp penalty on it which is overkill. Shield would mean not every class can use it. Armor would render the person so easily CCed its useless.
Name a class that can't use Guard [1]. 
If it was a shield then sins can't use katar, so they can't use their sqi with the card.
Also, sniper couldn't use arty, stalker couldn't use sherwood... Would be stupid and unfair.