Ok. The way I see it is the following.
The idea behind catapults is that they give the attacker the ability to crush walls making it easier to take castles. Problem now is, that the catapults are very effective at taking down walls causing castles to have little to no defensive purpose.
If you lower the effectiveness of catapults the attackers are just going to arm themselves with greater patience making the siegebattles even longer and drawn out.
Adding ammunition to catapult could be a solution, but is it conceivable that attackers would never be well stocked on this?
Right now, besides being a cool feature, I have a hard time imagining how catapults add to the fun of gameplay.
I might lack vision, but can someone come with a suggestion of how they can actually increase fun? Not just draw out sieges and after waiting for an hour making them pitched battles.