Missile troops. Now I'm not saying that chaos warriors need to start sporting bows or anything.
To my way of thinking marauders wouldn't really be the farming type, more the nomadic hunter gather type. So wouldn't they have bows for said hunting.
Now I know they're not most intelligent race in the WHFB world but come on even the thickest twit would eventually realise that the same bow that killed dinner could kill that Empire swordsman over yonder.
Of course anyone willing to live in a northern snowy climate probably isn't to bright to begin with, hmmm................
It explains in the book that they dont use missile weapons because they want to see there opponent die close up.