Night attacks are useful if you only want certain people to partake in the battle. Only those capable of fighting night battles will appear on screen (which means you can also lose out).
In a way its nice to get a command boost if the enemy general does not have the night battle ability (otherwise its pointless in a way).
On the other hand, you can make battles tougher for yourself if your enemy reinforcements are capable but yours are not.
Note that winning a night battle with a heroic victory will make your enemies be afraid of the dark and lose a command point in night battles in the future, but usually their general tends to bite the dust during battle anyway, so yeah.

AI can cheat with night battles since even its capatains can lead night battle attacks (but not defense).
Units are just as effective as during the day.