Peasants can be way more useful, delivers more variation in scene editing (The deathtrap in citadel would just not be the same), more variety when it comes to the whole siege experience, you can shoot other ranged or cavalry with them even as melee. The only downside is crybabies crying about getting oneshot, which could happen without the ballistas just the same, except you can't carry around or run away with the ballista. And unless you get shot after every spawn without even reaching any enemies, i seriously don't see where this frustration is coming from. It's siege, you respawn and move on.