It's not just because anyone is out of ammo, running down things is fun. BTW in case you haven't seen it yet there is also a perk that increases collision damage. You are right though the bosses are unaffected, anything lesser though can be run down squishified and eat tread, yeee haw! we done got us some mudders and bon-E-fied shooty guns! Git Some!! Oh hey, where's the six packs or moonshine at?