For a thumb-safety to be any good, the shooter must always operate the thumb safety any time the gun is picked up to shoot--even when target-shooting on a range where it would otherwise not be necessary. In a crisis, one is apt to rever to what one practices the most, whether or not it makes sense. If one normally picks up the firearm without switching the safety to "fire", that's what one's likely to do in a crisis, with bad results. And if one normally sets down a firearm without switching it to "safe" [nb: many semi-autos can't be switched to "safe" when the slide is locked back] one is apt to do so immediately following a crisis shooting, also with bad results. With the Glock, there is no reason to handle the gun on the range in a manner significantly contrary to what would be necessary in the field. Provided one always makes sure one's finger is off the trigger before lowering the weapon (whether the slide's locked back or not), and provided one doesn't try to collect brass after each shot, safe gun handling on the range will equate to safe gun handling in real life. Out of curiosity, does anyone know why John Moses Browning designed the 1911 so the slide cannot cycle with the safety on "safe"?
Operating the manual safety is a learned procedure, just as not pulling the trigger until you are ready to shoot.