My Sig has a rail and I have the TLR-1 light for the firearm. There are holsters that accommodate the light (Blackhawk in leather). However, I do not see a need to carry my firearm with the light. I work and live during the days and sleep at night. I leave the light on my nightstand and often put it on the light when I go to bed. I have practiced with the light a few times and for the most part that is enough. I also have a short shot gun in the bedroom closet and a Surefire on the nightstand as well.
So unless you think you are going to actually use a light with your firearm you are adding weight and bulk unnecessarily. At least that is my opinion.