I was actually having a discussion on this topic yesterday with a co-worker. He recently got some training and was told that it is better to have a handheld flashlight when wielding an handgun, rather than having it attached to the weapon.
The reasons explained to him, which he related to me, were in line with what was discussed thus far. He was told that having the light on the gun means that where ever you are shining that light, is also covered by your muzzle.
This could lead to a firing your weapon in a knee jerk reaction. Now, a lot of people will claim they are highly trained.. but you just don't know until you are in that situation. Your adrenalin is most likely soaring, you might have just woken up from a deep sleep.. everything is happening very quickly. You eyes are adjusting from the dark shadows to the bright light of your weapon mounted flash. You turn the corner and see an intruder.. you fire... But wait.. was that an intruder or a family member. To late.
He was told that first and foremost, the best thing to do is hunker down and call the authorities. If you absolutely must move, then have your flashlight in you off hand, raised high and out from your body and have you firearm tucked closer, and lower, to your body. You lead with the flash light. If an intruder comes at you out of the dark, they will meet your flashlight wielding arm first, giving you room to manipulate your weapon in your other hand. Having the flash light in the other hand allows you to scan the area without covering everything with your firearm.
This is what he was told, then told me. I don't claim to be an expert, just relating what we discussed.
I will say this though. I am a fairly good shot. I go shooting all the time. I read all the discussions on tactics and "what would you do/what should you do". Although I never had professional and ongoing training, I was fairly confident that I would be cool and collected if a situation should ever arise. I, in my mind, thought I was prepared.
Well, last week, I was sound asleep.. that really deep "I'm drooling on the pillow" sleep.. and the house alarm went off. I'm in the master bedroom on the second floor.. my three children's bedrooms are also on the second floor. My nightstand safe with my .357 is within reach of my bed. What do I do? Slingshot out of bed and bull-rush down the stairs to see whats going on , still half asleep. I took the steps three at a time and only realized I didn't even grab my gun when I got to the first floor. This happened in seconds.. my wife didn't even sit up in bed yet and I was out the door like a mad man. Yeah... that was a big eye opener for me and shattered any false sense of being calm and cool when the shoe drops.
The alarm turned out to be a faulty window switch. Only reason I related that embarrassing and truthful story is to bring to the forefront that.. you just never know.