When looking at a system that you can use daily, people like to account for as many variables as possible. In my case, having been through a good variety of shitty situations overseas and in the states, I keep it simple and prioritized. In the case of the subject mentioned in the title, I have only a few scenarios where having a light attached to your carry gun is necessary. I have fought at night and I can tell you that it is never a good idea to flash off a high lumen light in the pitch dark. But how to use a light is a subject for another time.
Usually the subject of a weapon light on your carry gun is brought up in a subject about EDC and the excuse of being prepared, JUST IN CASE. Now me being a master of sarcasm and happy to point out extremism, I would suggest that we also carry NVGs, full body armor, and camouflage utilities because the Russians may invade too. You just never know and you gotta be prepared, right?
Now sarcasm and all aside, the concept of carrying a light on your carry gun is a sound one, but only in a couple of circumstances. If you are going hiking and it is possible that you may be there overnight, then sure bring a light to put on your gun, but I wouldn't put it on until dusk. The other circumstance is if you are carrying here in Alaska during the winter. Anyone that has lived here knows that the winters here provide only about 5 hours of light in some cases and even at the height of the day, the winter clouds can make it low light conditions all day. Personally I just use night sights on my guns. Past these two things, I can not think of a sound reason for people to carry a light ON their gun. I have nothing against having a light, or even having one that goes on your gun, but carrying your gun with a light already on it during the middle of the day just makes you look ridiculous. I have noticed that the majority of people doing this are the ones trying to appendix carry a Glock 17 with a tac-tickle 33 round magazine in, just in case zombies turn up and Rick needs their help.
I know that we see some people who are instructors in the tac-tickle community carrying lights on their guns, and a lot of people think this looks so tacticool and want to copy it because this must be how to operate like that operator. Sometimes these tac-ticklers will bring up the subject of why they carry a light on their carry gun. Usually they will go through the difference ways to shoot one handed and use a flashlight with the other hand, and then show how easy a gun mounted light is in comparison. Then they conclude with just saying that it doesn't hurt anything. Okay, sure, it doesn't hurt to be over-prepared, but I guess that just means we should all just deploy our airbags in our vehicles when we buckle our seat belts, because it doesn't hurt to be too prepared for getting in a car wreck.
If you want to have the capability to fight for your life in the dark, carry a flashlight. Yes you can even carry a weapon light and put it on your gun when the time is right and it gets dark. But carry a flashlight on your gun if you are truly scared that you are in Silent Hill where day becomes night in seconds and your only surrounded by enemies. I will stay here in reality.