Well the big thing is, the enemy (and you, possibly) can't use any reinforcements, so it's ideal for picking off a small stack when a full one is lurking behind. Though in Medieval, I think, if the reinforcing general also had the night-fighter ability, they could reinforce. Less of an issue here with fewer generals having the ability.

Apparently all units get lower visibility, but I don't how this works exactly - is it just that all missiles get a penalty? You can still (I think) see all the enemy units on the map like normal, but maybe I'm imagining that.

Btw, has anyone ever had the AI force them into a night battle?