How effective was Roman artillery in Field battle?

[Valerian Pertinax]

can't readily recall many instances where they were even mentioned during a field battle, let alone "decisive", but this may just be that their presence was either taken for granted, or not of as much use as the first ten minutes of Gladiator would have us believe

Or maybe they didn't use them in places where they lacked giant amphorae of flaming oil and spherical hay bales?

Even if you don't have the giant flamey hay bales or exploding ballista bolts, I see no excuse to not at least light your pila on fire before throwing them.