Using flaming missiles with onagers is a quick way to wipe out a good chunk of any enemy's force. I find that the number of casualties and panic the flaming missiles cause more than compensates for the decrease in accuracy.
A normal onager missile usually inflicts two or maybe three casualties on a hit to a unit whereas a flaming missile can cause as many as 25 or 30 casualties if you nail some poor slobs packed into testudo formation.
It's a shotgun approach, but as yak said it works really well if you have massed or lots of enemies against you as you don't need to hit right where you are aiming in order to inflict casualties.
I've also found that you increase your likelihood of hitting enemy units if you're firing at an angle to an enemy column or across it rather than straight into it as misses are most often too long or too short rather than too far to the left or to the right.