I bet we have to partially blame Ridley Scott for the historical inaccuracy of artillery in Rome and Medieval II - after seeing those catapults in The Gladiator, who doesn't want a battlefield with strong artillery.

Seriously, though, as a game balance issue I think they have to be useful enough for lugging around the battlefield, as they limit the movement of the army hugely and their support cost is relatively high. Good thing that most arty can't do such feats as Onagers used to in Rome.

They are fun when you're operating them, but get annoying when you have to wipe out 10-12 of any siege engines that the AI insists on mass producing.