The only way I could use them in a field battle was to attack a mostly stationary target such as a huge clump of warbands trying to break through my line. I had one ballista unit and it took out an entire line of troops which still wasn't a lot considering I had to do it from close range and ended up getting charged. But 16 is better than 1,2, or three. I figure you have to aim for soliders trapped in natural bottle necks or against a wall of men rather than of stone. It would be stupid to wait for the enemy to stay in one place.