Ribaults are primarily an anti-morale tool, as the morale hit of being shot by them is very nasty. I like to park ribaults near the end of my line and let them take one shot as the enemy comes in, then order a hold fire. Once the lines are engaged instruct the ribault to fire along the line of the enemy's engagement, and you can nearly guarantee that whole wing of the enemy formation will rout.

I don't tend to get a lot of kills with them, but they entirely change the shape of battles by routing otherwise orderly troops. If you can arrange to hit the line with a cavalry charge right after the ribault fires you will shatter even the highest morale troops. Also note that their morale doesn't go back up immediately, so troops that rally can often be scattered again by a simple cavalry charge.

If you park one at the head of a bridge, or just inside a city, you've got a lock. The AI just can't get by it, providing you don't let it get overrun.