Well, even a fraction of their numbers is still quite a large number-- let's assume that each peasant unit has 120 men, and there are twenty of these units. EVen if only 20 men got shot before a unit routed, that's something like 400 men. A lot of javelins/arrows. I don't go out of the way to buld them-- they are just extras, so any advantage I can glean from them I perceive as a bonus already.