Because there is nothing more retarded than carrying around 3 empty stacks of jarids, each weight 4 pounds(?). Or arrows that weight 10.
Even worse is, that if I pick up a melee weapon with my thrower once I've used all my ammo. And later on loot some new kind of throwing weapons, my char will drop the recently looted melee weapon instead of any of the empty stacks or throwing stuff....
So, once the ammo of 1 stack reaches 0, the stack is destroyed. How about that? Pleeeease?