I have night sights and lights on three pistols. Those are my home pistols. My EDC just has night sights.
Having said that, both are good to haves, one or the other is a necessity to have. Lights break, batteries go bad, or you're not where you can grab one with a light when you need it.
I know folks who say they don't need a light because they have night sights. Some say a light is bad because it gives your position away. I know some who say that night sights alone are just crazey because if it's too dark to see the standard sights you can't be sure what you're shooting at.
Well, if someone is shooting at you, do you need to ID their face? Or do you just need to be able to put your night sights on their torso? Sometimes you may get enough light from another source to identify a threat, but still not enough light to line up those sights - unless you can see them.
The light does make it nice, to identify stuff when you don't have any other way of seeing what's there. You can use the momentary on/off lever/setting so it goes off quick if you need it to go off as you un-ass the spot you where in when you hit the light and saw a reason to duck/dodge.