I think you're missing my point here. The entire reason anyone would get a mod that removes invulnerability from quest givers is to... well, kill them. Why get a mod to kill quest NPC's if you're going to want to do their quests later on?
Pretty simple really. A mod that removes invincibility from all the NPC's that are immune. When I go to slaughter an entire city I want to be able to kill everyone. Quest-givers and children included.