Welcome to the NFOA Forum will102882.
Night sights can assist you in low light but they definitely will not help in a no light situation.
Tactical lights (good ones) while being some what costly are a necessity be it weapon mounted or hand held.
Some statistics state that 70% of armed confrontations are in low to very low situations.
With a frame (picatinny) mounted tactical light you of course will need a different holster for fit of handgun and light.
If intent on carrying with light attached.
There are a few examples of tactical light use both gun mounted and off hand.
Below is a god article to read up on one writers opinions.
http://www.allbusiness.com/crime-law-enforcement-corrections/law/11733798-1.html.Personally I use a Sure Fire flashlight hand held for my handguns. My Rem. 870 setup has a weapon mounted pressure switched tactical light and pressure switched laser. The light also has a fast strobe feature which disorientates the bad guy.