An exterior safety on a pistol is something one can refer to as a "proprietary to the user" safety, meaning, should someone other than the user get his hands on your gun, he may not be familiar enough with it to immediately shoot you with it. I knew one police officer that worked around here who carried his S&W Model 39 on safety. He was jumped,, beaten down, disarmed and left for dead because by the time the punks had figured out and flipped the thumb safety on his gun, they had also pushed the magazine release and dumped the mag, which bounced under their car. The M-39 also has a magazine safety, so the gun can't be fired when there is no mag in, regardless if the chamber has a round in it. The thumb safety saved his life, in this case.
Wouldn't happen to you, someone getting your gun? Don't be so sure. One in five American cops are killed with either their own gun or the gun taken from another cop.
Miami's P.D. did some research on this in the 1970's. The rangemaster grabbed people wandering their building and took them into the range. He had a loaded .38 Special revolver and a Colt 1911, loaded with the safety on, sitting on a table and a target hanging a few yards downrange. He told them, at the signal, to pick up the revolver and shoot it. The average time was under 2 seconds. Same drill with the 1911, the average time was 17 seconds. Sure, some of the shooters knew what a 1911 was and got the shot off quickly but others never figured it out and never got off a shot. But there was an average difference of 15 seconds. 15 seconds is time enough to put a second plan into action. If nothing else, even at my age, I can run a long ways in 15 seconds!
Some think a thumb safety will slow you down. It shouldn't. It takes much less time to the flick the safety off than it does to draw and raise your pistol, and since you can do both at the same time, working the safety while drawing the gun, it shouldn't slow you down at all, unless you are incredibly ham-fisted, in which case...
It's a simple device, a thumb safety. It takes very little practice to make it part of your draw stroke. Do you have to remember to put your finger on the trigger when your muzzle covers the target? Do you have to remember to move your foot from the gas pedal to the brake pedal when you want to stop your car? No, a little training and practice make it natural. Learning a thumb safety is in the same class of difficulty to learn. It's not rocket science.
Does this matter? Mebbe, mebbe not. What will YOUR gunfight be like?
In any event, you can have a choice in the matter when you pick your sidearm. Choice is good.