I just got my first siege, and I think it'd be an awfully FUN addition if colonists would be scared out of their minds while the base is being shelled. That's all.
An "We are under attack!" mood debuff would be sufficient, and colonists with psychopaths/bloodlust could get a mood gain from it!
This is why the bugs wont keep me from my mountain. All safe from the shells there. Of course there should still be a -1 per shell that lands and have them stack.