They are pretty crappy, I built them once and then never bothered with them again.
Dang! I was really hoping that they had some kind of use. I really like the idea of weapons particularly designed for city siege or defense. Perhaps they could be buffed and their movement reduced to 1? That way you would more or less be forced to use them for city defense, which they should excel at, or city sieges, but it should take you a long time to wheel them to the front. Historically, siege weapons slow an army down to a crawl because transporting them is overly difficult. Maybe then you would have more motivation to split up your armies: one or two to go ahead and engage the enemy forces, and one to protect your supply line and siege units. Of course "supply line" isn't a thing, but you get my idea.
One simple idea that comes to mind is giving the catapult a chance to knock down and/or stun units in the AoE of its shot.
I like this idea! You could possibly also force them to be stationary on the battlefield: after all, once in place, siege weapons (even catapults) are notoriously difficult to re-position in battle, due to their size and general unwieldiness.
I'm hoping for a weaker type siege engine at a midway point in the tech tree.
Yes, I think this is a must for siege weapons to gain some sort of presence in city sieges.
While we're at it, I would like some sort of Stone Age bow, too. If only their were some sort of "hunting" technology which gave you weapons cross-tree (it would be a civilization tech).