The stats for peasants are c 4/a -2/d -4/a 1/m -2. This equates to a unit with a decidedly average charge and very poor everything else. There is simply no reason to commit peasants to battle in MTW - but the AI seems to think otherwise. The only solution IMHO is to mod them out of the game. I have played MTW with peasants modded out for years and could never go back to fighting battles against peasants again. In my games peasants only appear occasionally in rebellions and not otherwise.

When it comes down to it even vanilla archers (c 1/a -1/d -2/a 1/m -1) can beat peasants in melee. The peasants may get a few kills in on the charge, but after that it's in the archers' favour.