OK, I get that. Still, if you have a handheld light, what's the purpose of the one mounted on the gun?
The one mounted on the gun is for shooting and when you can point your gun at something. It allows the user to use their support hand for other things (calling 911, using a radio, herding kids or other uninvolved people out of the way, picking up items, etc) while still allowing use of the light and weapon.
Law Enforcement operates under some different circumstances than the home defender (not different rules) in that when we clear a house or structure, we have generally made announcements that we are law enforcement entering and for anyone inside to make themselves known. Due to this we can form a reasonable expectation that anyone inside who is refusing to come out is a threat. Thus, operating with the weapon-light is more applicable.
For the home defender, we don't know if the "bump in the night" is a home-invader or our kid getting a snack. Therefore, utilizing a weapon-light as a primary means of identification is probably not ideal. Of course, all my family's bedrooms are together on the second floor. If I hear a commotion downstairs, the first thing I am going to do is confirm my family is safe and consolidate them. Once that is done, I have a reasonable belief anyone else inside the house is a threat. It's not a confirmation, and we must still identify threats, etc., but in that case, in my opinion, a weapon-light is justified. Even so, Just like i carry a spare mag with my carry gun, I would like a spare light. One well known trainer who is big into weapon-lights uses a lanyard with his hand-held so he can transition quickly to the weapon-light if necessary.
Another area where the weapon-light becomes a big benefit is in a reactive situation. A weapon-light is much faster and consistent in activating and shooting than obtaining a hand-held light/pistol grip technique. In gunfights time is of the essence and our ability to perceive what is going on, make a decision, and act on that decision drives the speed of our response. The faster we can observe, orient, decide and act (OODA loop), the more likely we will survive.
Further, I'm not a fan of having to use your trigger finger for anything other than the trigger. If you have a light that can be actuated with a normal firing grip, then OK. Otherwise, like the TLR-1, I see it as a detriment to operating the gun.
I claim no expertise here. These are just my observations and thoughts.
The two main weapon-light families (Surefire's "X" series and the Streamlight TLR 1 and 2 series) both have pressure switches that are unobtrusive and instinctive, though the Streamlight version is only available for Glock and S&W right now (unless they came out with new models very recently). I agree that the trigger finger should be used for the trigger, and that's why I'm a big advocate of the pressure switches (Surefire's DG switch and the Streamlight Contour switch). This way simply gripping the gun in a normal firing grip activates the light.
When using the factory toggle switch, we train, as do almost every other trainer I've seen, to use the support hand thumb to manipulate the switch. While this obviously works with the thumbs-forward grip, other grips don't work so well and that's were the pressure switches (or training in the more efficient thumbs-forward grip) becomes pretty-much mandatory.
The only time I teach trigger-finger manipulation of the toggle switch is in wounded shooter drills when firing the pistol one-handed. In this case, the toggle is activated into the constant-on mode and left on so that the shooter can concentrate on handling the problem, not manipulating a light.
So yes, I agree that manipulating the light with your trigger finger is not ideal, but if you have only one arm, you can't use the hand-held light and weapon at all together, so a "less-than-ideal" use of a weapon-light is still an advantage over no light at all.
