If we want to be realistic, catapults shouldn't be used against troops in the field. It very rarely happenend. They were almost exclusively used for seigecraft. Subudai used them to capture the bridge at Pest, but that was a genius and there was a fixed target -- in real warfare, it was pretty rare to use them except to defend fixed points because they were too cumbersome to move. They were usually built on the spot for seigecraft.
So, please, don't say 'catapults could defend themselves' or 'catapults can't defend themselves', neither is realistic. When they were used, they were integrated with the army, they weren't 'seperate' units as we think of seperate untis, and they were bombarding fortifications.
Now, from a game standpoint, that's different. Catapults are VERY powerful. Realism or not, the game needs them because of the way combined arms works.
Say you have a stack of two axemen, two horse archers, and two spearmen. We all know the drill -- its very hard to attack this stack. If I have the exact same 6 units, whichever one of us attacks will get annihilated because the best defender will be picked in a rocks/paper/scissors exercize. The defensive advantage would be tremendous even without the terrain or fortification advantage.
So, attacking is hard. But the catapults change that. Take that same stack of 6 units we each had -- if we each had on catapult, the defender still dominates. But give us, say, 3, and now the attacker can win. Attack with three catpults, one or two may even survive, and the defenders are worn down to the point that you can attack. If you put 5 catapults in each stack, then the attacker has a big advantage. Attack with the 5 of them, you probably lose 3, maybe only 2. You can then take the stack of 6 units down with about 1 or 2 losses for a large victory.
So, when attacking, lots of catapults make sense.
So, we really can't have realism. Having lots of catapults in a field battle only happens in Hollywood, not on real battlefields. I view the catapults as a way to have unit type advantages (a good feature in my book) without having defense being totally dominant. They aren't even close to realistic, but when I play CIV, I play to win. I probably build about 1/3 - 1/2 catapults when attacking, and I understand that the stronger players build even more.
Best wishes,
Breunor