The Shield Problem(s)

[Musashi]

Most factions have at least sergeant spearmen from very early on in the game.

Additionally, even if a unit can't beat another unit, there's still often a reason to bring it. No matter how weak your best spear unit is, you'll always bring a few, because heavy infantry, even really good heavy infantry, can't hold a charge. They'll be annihilated. Often you bring your spears just to die, but their merit is in the fact that they die slowly, giving your real worker units a chance to flank and kill the enemy cavalry.