Hmmm... human quiver/pincushion, as in purposefully draw the fire of archers and/or towers ? push battering rams around, assault the walls just to tire the ennemy before the real thrust ? Bog down cav for the spears to charge ? Artificially pump up your forces so that you're higher on the military scale and your neighbours are scared of you as long as all they see is your professional borderguards ? Entice the enemy to chase them on the campaign map so that you can take their defenseless cities ? Use them to cut land trade and cause devastation ?

I can think of a lot of things a useless peasant can be used for