The best use for all gunpowder troops in field battles is morale damage. Kill the opposing general, fire through his men as they advance, and by the time they reach melee range their morale will be low enough for one good charge to cause a rout. They will not kill an enemy army for you, they'll just make it easier for you to break them. The ribault is particularly useful in this capacity as it won't collect many kills, but it will scare the heck out of the men it's firing at.

In the campaign they're obviously very useful in sieges as well.