I prefer safeties on my guns for those one in a million times when something happens you didn't expect or plan for. I've seen several negligent discharges by people who swear the only safety you need is between your ears. My first duty gun was a S&W 59 and I went through the academy firearms without a single safety problem. I've never had a problem flipping the lever off during the draw. There really is no reason not to have one since there's no down side and always an upside. It's funny because I've personally known three "gun guys" (two that were LEO's) that were part of the "only safety you need is a brain and a finger" club that not only had negligent discharges but shot THEMSELVES doing it. To this day if you bring up "the only safety you need..." subject they will have episodes. They said the same things I'm reading here and proved that even a highly trained and well disciplined gun owner can make a mistake.
I love my Glock 21 but I do not like a safety that is mounted on a trigger. Every negligent discharge I've witnessed was the result of a finger or something else pulling the trigger. Putting a safety on the trigger doesn't do a thing to mitigate discharge caused by the trigger being pulled. Its why my Glocks all stay in the holster, covering the trigger, when not being shot, personally I think Glock should have put a grip safety on their guns too. I like having that disconnect that ensures, if a finger or a stick or a string or anything pulls that trigger when it's not supposed to be pulled, I'm not going to have a bullet flying out and all the dangers that entails. If someone else is comfortable with the risk that's fine too, it's their choice but saying the only safety needed is your brain is living in denial. It may be all you need most of the time but that one time out of a million when someone is accidentally injured or killed proves its not "all you need".
No offense, but LEO's aren't exactly what I would call well trained in firearms safety, most of them never touch their weapons for anything other than an armrest and annual qualifying.
Quality carry holsters cover the trigger negating the need for a manual safety.
Saying that anything other than your brain can keep you from discharging your firearm negligently is living in denial.
Discipline and respect for safety are all that one needs to keep from negligently discharging their firearm.