Your fish have no benefit from a nightlight. They will still be able to see. If you would like a moonlight for aesthetic reasons for yourself, by all means add one, I've run them before. Just don't over power the tank with them.
What Joe is pointing out is that if you run a light on your refugium/sump 24/7, or at least on the opposite schedule of your main tank lights, it will help keep the water parameters, specifically pH, more stable. With a tank that only has lights on the display, after they have been off for a bit, the algaes that change co2 to oxygen stop converting. Yet your fish and other organisms are still using up oxygen. If you've never measured your tank's pH in the wee hours of the morning, do so. It drops more then you might think.