I have 2 SA .22LR range pistols with super light triggers and they aren't even drop safe. They have manual safeties. I NEVER use them. When I want to shoot them, I load them. When I'm done shooting them, I unload them. Even if the manual safety was on, they still wouldn't be drop safe. When I carry one in a holster, it's with an empty chamber. I have never had reason for either of these guns to leave my shooting grip with a round in the chamber, with one exception - clearing a jam for a friend and passing the gun back, muzzle pointed downrange.
Have I ever pulled a trigger meaning the gun to fire, but accidentally had the safety on? Dozens of times.
Have I ever accidentally pulled the trigger when I didn't want the gun to fire? No.
Does a manual safety have a place? Yes. IMO, there at least 3 good reasons to have one.
1. Holster substitute. The only handgun I'd keep chambered and lying around unholstered is a gun with a reasonably heavy trigger and/or a manual safety, and complete drop safety. I do not leave a loaded and unholstered Glock lying around, but I do that with some of my other DA pistols with manual safety. That is a useful feature for this purpose, whether you want to have a gun on a nightstand or center console, or w/e. It takes 2 hands to remove a gun from a loose retention holster or to rack the slide, and only one hand to pick up a loose gun, acquire an appropriate grip, then finally flick off the safety. Adding a good, stiff manual safety to a Glock would give it this one benefit at the cost of increasing the manual of arms when you are not doing this one specific thing.
2. If a gun has a very light trigger pull and you intend to keep it chambered. A Glock trigger is lighter than most DA revolvers. But the gun is lighter, too. For me, the trigger weight must be heavier, as the weight of the gun goes up. It's physics. If you mishandle or bobble a heavier gun, the trigger can get pulled harder. Also, the huge triggerguard on a Glock is a safety feature. Most revolvers have a very thin trigger guard. I think Glocks are just as safe as a typical DA revolver.
3. If a gun is otherwise not dropsafe, but the manual safety is there to make it so. Of course, it would be preferable if the gun were drop safe even without the manual safety being applied, but not all guns are designed as well as a Glock. The only example I can think of is my P-64. The manual safety is the only thing that blocks the firing pin, which is why I use it even though the DA pull is 25 lbs! Or course, I'd want to cite the 1911, but most are not completely drop safe even with the safety on, though the safety helps in this regard by locking the sear.
