In my opinion, units using two-handed-swords should get the armour-piercing ability.
1. Balancing reason: Without it, these units are useless; They are very expensive, come quite late (HRE: citadel baracks) and lose consistently to older castle units (Dismounted Imperial Knights, Dismounted Noble Knights (?)).
2. Physical reason: The armour-piercing ability is a matter of pressure. Pressure depends on the force and the area. The heavier and stiffer your weapon (assuming constant collision speed) and the smaller your area of impact, the higher your chance to penetrate the armor. In most cases, it is enough to make a deep dent in the armour because that hurts the wearer just as much as penetrating would hurt him. Twohanders are used as blunt weapons just like axes, they have the weight of axes (or more) and they can inflict serious damage on an armour. Therefore, they should be considered as armour-piercing.
Your feedback will be appreciated.