Erick bringsup a good point, it is hard using a supplemental light when working a two-handed weapon such as a shotgun if the light is not part of the shotgun. The closest I have come is putting the light in my teeth and depressing the switch with my tongue. Of course if the bad guy goes for the light source, then he will be shooting me in the head - not a good option for clearing a house.
Hand held and weapon mounted lights, for most folks not doing dynamic entries into buildings foreign to them should probably only be used as supplemental lights. On a home defense weapon, I think a lot of folks get the impression that since it is their home, searching in the dark using a flashlight or weapon mounted light will give them the advantage over a bad guy who does not know all the hiding places and hence is sort of trapped by the darkness and fear of running into walls, tripping, etc. That is crap. It is NOT a good idea to search your abode using a flashlight or weapon light as the only source of lighting. As noted, if you don't immediately spot the bad guy, he will immediately spot you and then you have lost your advantage.
Where possible for home situations, if you insist on searching your own home, make use of available overhead lighting when possible. Overhead lighting tends to level the playing field in that it takes away the low light factor from both parties.
So when do you use a weapon mounted or hand held light? They are best used for supplemental lights and not a primary lights for the average civilian. They are great for checking closets, around the back yard, the attic, and just dark corners in the house. They are great for peaking inside your parked car or under it. Use them where normal lighting is not very good.
What makes weapon mounted lights so useful for tactical officers working in low light conditions is a combination of factors. They will tell you that given a choice, they would rather not do building searches in complete darkess using only their lights. Why do they use their lights? That answer is two fold. First, unlike most homeowners, they are not doing a building search by just one individual. They are using group tactics and essentially swarm the building. While one officer is looking one direction and sweeping the area with his light, another officer is looking in another direction with his light. A big disadvantage of being single with a single light looking in the wrong direction is essentially removed since there are multiple officers and multiple lights covering multiple directions. That helps to preclude being attacked by a person coming out of the darkness where you don't have your light pointed. The second major aspect is that officers do not have time to stop and look for light switches to turn on overhead lights and even if they did, there is not reason to believe the lights will be functioning. Control of the structure is secured via speed, getting through as clearing as many rooms as possible as quickly as possible.
Weapon lights are perfectly fine for home defense guns, but they need to be used properly and more often than not, they should only be used as supplemental light sources and not as a primary or solitary light source as they are quite limited in their usefulness with their highly directional illumination.
