Body Armour on Shields reduce damage dealt to the shield itself. Use high armour shields against low base impact weapons (like arrows) and you will be able to have a practically immune shield, where as use high HP shields against weapons that do high base damage. This is why a board shield is king against a Danish, where as you would want a steel plated shield against a sidesword, and never the other way around.
Shields on back grant some protection against projectiles only.
