Tueller Drill is a good reason to carry with one chambered.
It was determined that the average attacker could cover 21 feet and stab the defender, the time was 1.5 seconds.
Now, if you can clear your cover garment, draw your weapon, charge it and get out of harms way in that 1.5 seconds and successfully engage the threat, before the threat can react to you then you stand a chance to survive otherwise, you are defensive with a club while he is slashing and stabbing, not the place you want to be.
I carry in a Thumb break holster (If I want to carry on company property, their rules) I can clear my cover garment and draw in 1.5 seconds from a Don Hume H726, with a lot of practice and get off a near contact shot while moving to avoid getting a knife in the chest, we used 3D targets mounted on remote control vehicles to simulate an attacker. More than once there was a collision, requiring the use of my left arm to prevent a stab to the chest or actually take a stab to the forearm (we used rubber training knives with chalk) to allow me to push off the attacker to engage, this means that many times my left arm was theoretically out of commission and the attacker was in my face and given that we weighted the target (150 - 180 lbs) this wasn't like having a piece of plastic run into you, it was actual physical mass.
The recent shooting in Dayton, Ohio is another case in point.
2 on one with the victim having his pistol secured in the center console of his vehicle, he was using one arm to hold himself in the vehicle while attempting to get his pistol with the other , in his words there would have been no way for him to have charged the pistol if it had been carried with the chamber empty, he got his pistol and engaged the threat, threat ceased to be one and the rest is a foot note in history.
On letting a child go basically unsupervised to retrieve "ANYTHING" from a gunlocker NO! NO! NO! NO! NO! "A moments inattention can lead to a life time of grief" as my dad use to say, you were lucky.