Depends for me. If I can carry concealed outside the waistband, then I will carry with the light attached to the gun. BUt can generally only do that in the winter. I do have a custom IWB holster on order thaat will work with my mounted light, and will use that once I get it.
I did a low light defensive pistol course a couple of years ago. I was the only one using a weapons mounted light. At the start of the class lots of guys were talking bad about the mounted light. Then we started shooting, and they saw how much easier I could get good hits, because I had both hands on the gun. I was also using a Streamlight TRL4, which is a light / laser combo. Huge advantage in the dark. The guys talking bad about my light in the beginning, were wanting to try mine out by the end of the class.
One big advantage I was able to show during the class, was simulating covering a target with my weapon in one hand, and being able to call 911 with the other, and still have the light and gun pointed at the target. No one else could figure out a good way to do that.
I also carry a back up light too. 1: If the primary weapons light goes down, I have a back up. 2: Sometimes it's not appropriate to pull a weapon to use the light thats on it.
Every long gun I own has a light on it.