Piercing Damage

I'd be happy to elaborate to anyone who wants a deeper explanation (I'm pressed for time at the moment, what with classes tomorrow), but the skinny of it is that unless armor is hit with enough pressure to break the bonds holding the steel together, the pressure of the blow is distributed across the armor.

The implication is that if we assume no human can swing or thrust a weapon hard enough to break steel (assuming weapon and armor are made of similar materials, this is a fairly realistic assumption, even in Exalted), it's next to useless to stab an armored body area with a bladed weapon, because very little of the armor experiences the initial force, leaving most of the armor's surface area ripe for the force of the blow to be spread out over - this final, spread out force is what hits you.

For this same reason, a blunt weapon such a grand goremaul hits a much larger area, leaving less armored surface area across which to spread out the blow. The spread out force (the one you experience) is much greater than in the case of the stab. You'd have slightly better luck slashing with a bladed weapon, but as most of these are made to cut, the edges are also very thin, meaning you're delivering the pressure over a small surface area, which is disadvantageous unless you literally rend the armor, which you probably won't.

This is why large, blunt weapons do piercing damage - calling it piercing damage is something of a misnomer, though, because it has nothing to do with the weapon's ability to pierce. It's true that they are more effective against armor.

Target arrows are the outlier - these actually do pierce. Fired from a bow, they're much faster (enough so that they have, overall, a much larger momentum) than any weapon can be swung, and they deliver enough pressure that the bonds holding the armor material together break. In this case, the kinetic energy of the arrow goes into the tearing, and is not distributed over the armor.

This is where the dire lance, our classical talking point, comes in. Standing on the ground, it's increased length means it probably does a better job of piercing (more torque) than a sword would, but it's still fairly absurd to think a human could apply enough force to pierce well-made armor. When you put this human on a horse riding 30 mph, however, the size of the lance, torque of its length, and speed at which it is traveling make the rending of steel very realistic if build quality is similar between weapon and armor.

Thus, the house ruling is that dire lances inflict piercing damage when used during a mounted charge, and blunt weapons with piercing tags retain these tags.

Oh, as a corollary, riders aren't immune. Someone stationary using a dire lance against someone who is riding at them would also inflict piercing damage for the same reason.