I never use artillery units........I play vanilla 1.2 campaign and I rarely don't wan't to use artillery on the expense of other combat units. But I must admit that in some battles where you are the attacker and the enemy just sit back and wait, onagers would have been nice.

I have rarely experienced the AI use onagers until I (Jullii) was outlawed by the Senate and now face the other Roman factions. I have had several battles against the Scipii and they make extensively use of artillery. Yesterday I was fighting a full Scipii army with at least half of the units being onagers. This caught me completely by surprise and I was forced to rush my attack instantly........I won the battle, but it was nice to see diversity in the AI strategy;-)