It’s just depressing to see that archer firing his bow standing beside a perfectly fine cannon.
This would add an incentive to build siege weapons – they shound not despawn while there is a npc around. And it would allow to further and strategically upgrade keeps to prepare for the night.
The first trebuchet shot by a NPC should be a miss. Afterwards it can use its superior computerized aming powers (not including statistical movement prediction :-).