"...if safety rules are used..."
Yes, if a gun is in a proper holster, it is safe, even if it has a light, competition trigger. If a person, under ideal conditions, uses proper safety "protocols" with the gun, any gun is safe. That is not the argument here. Additional safeties, that prevent the gun from firing, help to take care of situations that you might not see coming.
The issue with negligent discharges is that people aren't perfect. If distracted, or if wearing certain types of clothes, a gun can discharge if the trigger snags. A few years ago, I read about a cop that had a negligent discharge because the drawstring on his windbreaker snagged the trigger on his gun when he holstered it. No frame safety, no grip safety. He had his finger off his trigger, but he didn't see or feel the draw string. He shot himself.
An activated frame-mounted safety or a grip safety would have prevented this. Sorry. Unless you are a cop, you are more likely to shoot yourself with your own gun than being shot be someone else. Any gun with a frame mounted safety that is engaged, or a grip safety that is not enaged will always be safer to handle than one that does not have these added safeties. If the gun can't go bang when the safety is on, the gun would be safer to handle than one that doesn't have a safety. Come on. This is not that hard...
This isn't about your superior training or discipline. This is a matter of an instrument being safer to handle. Heck, any gun with a hammer is safer to handle than a glock or an M&P. All you have to do is to apply pressure to the hammer with your thumb when you holster the weapon.