Well, I was trained to use a light in the off hand, holding it in front, as well as up and away from me, on the theory that if a BG shot at the light, he might not hit me. Now, it seems that not only do people want to put lights on their guns, they want to hold the gun directly in front of their faces. If I am a BG, hiding in a big area like a warehouse, and feel I have nothing to lose, I will shoot at the light and the cop behind it will be in bad shape, as in dead. And please, I have used very bright lights in dark areas, and there is no such thing as a gun-mounted light so bright it will blind everyone anywhere in a big open area. Anyone who thinks otherwise just hasn't tried it. And anyone who thinks he can turn on a light and immediately spotlight the BG is out to lunch. He may be in front of you, or off to the side, or on a balcony, or behind a counter, or anywhere in the place; the chance of lighting him up first try is about nil.
In fact, my instructors pointed out that if possible to do so, it is best to simply turn on the regular lights. Your eyes might take a second to adjust, but so will the other guy's and with the area lit up you are on even terms. (You don't just switch on the lights and stay by the switch - you move as soon as you hit the switch.)
Jim