I think we can all agree that if you can't clearly see your target, you shouldn't shoot at it. I think we can also agree that it is very likely that a violent encounter will occur in low-light (or no-light) conditions. That is why, in my personal opinion, guns should come with lights. At least all guns designed for fighting and personal defense should come with lights... 

I have participated in multiple low-light training events where many of the experienced shooters suddenly realize how dramatically defficient they are at shooting in the dark. They don't have good experience or acceptable skills to be able to effectively manipulate their offhand light and their weapon at the same time. Their times go to Hell in a handcart, they fumble their basic weapons manipulations, their shots and follow-up shots are slow and inaccurate, and their light seems to act as a similarly polarized magnet to their front sight and pushes it off track. They focus on the center of their light's beam only to find out that their front sight is nowhere near the target. Then they correctly get an acceptable sight picture only to discover that their light is now pointed somewhere off in the distance. They repeatedly "paint" themselves and their location as they try to manipulate their handgun during reloads, malfunction corrections, etc. This foreign object (flashlight) somehow destroys their skill sets that they thought they had mastered.

Then there's the other guy... The guy on the line that is shooting at his usual pace, correctly manipulating his firearm, reloading at blazing speeds, etc. He's performing up to par, and he's scoring good hits in the process. The multiple people in the first group are wondering "How in the Hell is that guy so good?"  Then they look down to find that he has the difference maker... He has a weapon-mounted light. Yes, this guy who is performing within tenths of a second of his usual par times has a light mounted to his firearm. He makes only minimal adjustments to his grip in order to manipulate his light. He still grips the firearm with two hands. He isn't fumbling around with a flashlight and a magazine in the same hand during a slide-lock reload. He has the advantage. 

In a gun fight, why wouldn't you want this advantage?

You have the option to employ a weapon-mounted light with a minimal additional investment. Instead of purchasing another "safe-queen" gun that you're going to fire 250 rounds through (total), make the investment to purchase a quality weapon-mounted light and a holster that will securely fit your firearm with the light. Is it a little extra weight? Yes. It is a little harder to conceal? Yes. Could it honestly make the difference in being able to engage your target effectively and not being able to fight because you never train with a flashlight in your hand? Absolutely. 

Note: this doesn't mean that you should neglect training your freehand flashlight techniques. It doesn't mean that you shouldn't be very competent and familiar with weapons manipulations with a foreign object in your support hand. It simply means that in a gun fight, we want every possible advantage that we can get. Every firearm that I have that I plan on possibly using in defense of myself or loved ones will always have a weapon-mounted light, night sights, and plenty of rounds that have been through it during training. I recommend that you do the same.