They do do that, but the reason that WFG chose to give spearmen pierce attack to begin with was because well... most of the time spears pierce. It's a case of making a thing intuitive on paper that is much less so in relation to the game. The reason I prefer melee, ranged, and siege is that there is little doubt about how these work, which I consider grounds enough to make it an objectively better improvement. Granted, hack, pierce, and crush are fine and perhaps good enough to not warrant bothering with a change, but it's because of these terms that the problem existed to begin with for vanilla 0 A.D.