In the good ol' MTW You can make a decent defensive formation out of just the basic Halberdiers, as they had a very high defence value (around 6) and could take cavalry charges pretty well, as well as kill a few horses while at it by counter-charging their charge. And as they had the AP ability, they were much better overall killers than spearmen, so they could damage old school spear walls consisting of Chivalric sergeants by just engaging in melee.

They weren't very effective killers compared to Billmen, the Swiss Halberdiers or the Jannissaries, but non-AP missiles just glanced off of them and it took quite a lot to take them down.

So the very basic Halberdiers were more of a defensive unit (slow, low attack score even with the AP bonus against knights) in my books anyway, while the other types were assault-oriented, and not easily available for more than a few factions.