charms are blades - enhanced damage. wards are shields - enhanced protection
Armor piercing weapons aren't going to dull someone's sword, they're going to so through the armor easier.
But see it like this:
Charms are weaknesses - Enhances protection. Wards are traps - Enhanced damage.
Armour Piercing weapons aren't going to dull someone's trap, they're going to go through the defences easier.
This is my problem. Weakness, pretty bad. Plague (especially Virulent) a nightmare in a team. But Juju can't be dealt with easily (ESPECIALLY NOT FOR BALANCE [where are our school-specific buffs KI?]) unless you know Cleanse Charm.
Even though they're essentially another layer of armour, pierce ignores them. Either enhance the stat or make another charm-specific one to consort it.
