I've noticed that except for the AI's ridiculous tendency to sit there and take it when defending, siege weaponry is mostly superfluous on the field. They'll do damage, and they'll wear down morale, but they are not cost-effective as field weapons. One notable exception is the bridge battle, of course, and that using siege weapons during one is a simple pleasure. Still, those are few and far between.

One note on scorpions, though: they're actually useful in attacking stone-walled cities. They do a helpful job of removing defenders from the walls prior to using siege towers or moving in past archers.