A few responses to various posts, presented somewhat randomly . . .
I carry a handgun everywhere it is legal to do so, and a light everywhere. The light has been a Surefire E-series 2xCR123 light since about 2002 or 2003, starting with an E2E, L4 (for more years than any other light), and currently E2D LED. I have not found the lights to be burdensome to carry, and have EDC'd Z-series lights before that time.
If you are carrying a handgun, one hand drawing, shooting, reloading, and malfunction clearing should be part of your skill set. If they are not, seek additional training. So, using one hand to manipulate the light should not be a disadvantage.
Tritium sights are quite visible, and even helpful, in sufficient light to identify threats, unless they are dimming out and in need of replacement. Complete darkness is not required in order to see them. If you are facing complete darkness, you may wish to use your light quickly to identify your target, and then use your tritium sights, without the light, to engage the target. You then want to MOVE.
It is impossible to predict the situation you will face in advance. You may have a reason to have the light out in advance, or you may not. You may have enough light without your light to make the decisions you need to make, or you may not. In any event, having the light with you is never a disadvantage.
Regarding comments about not carrying a gun but being trained in the martial arts, they are not mutually exclusive alternatives, but instead are both complementary pieces of the puzzle. The martial arts will be more useful in a wider variety of situations, including those not rising to the level of deadly force, or those requiring creating the opportunity to access the gun. Some situations, however, will not be resolvable without the gun.
Regarding weapon mounted lights, they have their place. However, if you are searching, do you want to cover everything you need to see with the muzzle of your gun? Probably not in most cases.