In my finished Scipii campaign I put 2 units of o(w)nagers in every stack I had (so 4 o(w)nagers on the battlemap). They are amazingly powerfull, especially when defending. You have to put them almost in front, just behind your first line of infantry. Then attack the middle unit of an enemy formation. Even if they miss, they probably hit a good chun of other troops. They never hit my own troops (you need to alter to another unit when your target gets too close, or even put them to halt)
In sallying forth it's even better. Put them behind your wall (if you have a stone wall, you need to guess the arc, otherwise they hit your stone wall.. not that badd, but if you position them just right, about 75% of the shots go over) and go shoot. The AI won't attack (probably doesn't even have siege equipment). So you rain hell on them. Once you are out of ammo, go through the gate and mop ub.

They are simply amazing for their cost. And no matter what, you need to kill them as fast as you can if the AI has one. I think he got lucky with his shots. Heck I wiped out a full 300 men army with only o(w)nager flaming rocks once... (the AI was waiting for reinforcements.. unfortunately, the main army was ash when it arrived..)