Me I'm very anal about my armies I could never waste a slot to more than one or two arty pieces that takes away space from more effective combat troops. The only factions that ever will train special roster siege armies for are Parthia and the Romans. And even then those armies only have 2 onager units. In a field battle they can be usefull if you set them to fire at will and hope they score a hit in the middle of a cohort.