Billmen simply aren't very effective. Their main use is as anti-armor troops on the flank of an already pinned unit. For example if the enemy has armored swordsmen and you have only billmen and spears you let the AS engage your spears and push the billmen in on a flank.

Billmen's defense value is very low relative to professionial infantry, so any unit which hits them head on is probably going to plow right through them. Cavalry head on will simply wipe them out. If you can get them on a flank against a pinned enemy, however, they can slowly chew their way through heavily armored targets.

Personally I'd rather have more spears and archers than any billmen.