Fixed direction artillery can be good in a defensive position, espcially if you move your units to the crest of a hill near the edge of the map. That pretty much guarantees the direction of the enemy attack, and you can place your guns accordingly. The artillery fire will diminish the advancing impetus and lower morale in the enemy so that when they make contact with your line they are already a little shaken and much more likely to route. I usually withdraw my artillery units after it is clear that the enemy has no more reinforcements coming on the field (this is where agents and watchtowers come in handy) and bring on more mobile units for the pursuit phase of the battle. Positioning is everything. If you are close to the edge of the map you pretty much know where the enemy is going to hit you and the time lag between withdrawing a unit and bringing a unit on is short because your lines are right on the entry point of the field, this is important for effective pursuit.
Fixed direction artillery is kind of a waste on the attack, or any artillery for that matter. You are usually started too far away from the enemy to use the artillery for preping fire and it takes up valuable slots in your line of battle. On the attack better to go with a strong missile unit and use them to soften up the enemy before your assault.