How effective was Roman artillery in Field battle?

[M. Demetrius]

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?