In my experience, catapults are only useful for defending your cities. A couple of spearmen can hold off a huge stack of enemy units if theyre backed up by a few catapults. The catapults will alow the spearmen to last long enough to become elite, and then things get even easier. In addition, they arent resource dependent so as soon as you research Mathematics you can build them in any small city too far away to reinforce effectively. Also, catapults upgrade to cannon. I used to rely mostly on spearman/archer combos for defense until I discovered the wonders of catapults. So to sum up: theyre attack value is neglible, but for defending cities from stacks of enemy marauders, theyre worth every shield.
edit: almost forgot, the AI quickly "learns" that city X has catapults and will tend to stay away from it. One more reason catapults are good.