I do not carry a round in the chamber when I carry a pistol (I actually prefer to carry a revolver, J frame, but that is not what the OP asked about).
I can get a round in the chamber pretty fast, if I see a pack of dogs coming, a rabid animal on the trail, etc. If I see a gang of evil-looking miscreants coming down the walking path towards me, I'll move off at a 90 degree angle to create time and distance.
If it is so dangerous that I might have to bring the gun into action immediately, without the one second buffering time it takes me to chamber a round, I should not be there.
If having a round in the chamber would save my life, one time out of a hundred, or possibly lead to an innocent person being killed, one chance out of a thousand -- I'll go without a round in the chamber. The right to self defense is morally defensible, but causing the loss of an innocent life is not.
I generally do not put a round in the chamber when I am deployed. I would put a round in the chamber if I were in immediate danger -- driving around, or training Afghans, for example.
What am I afraid of with a round in the chamber? Well, a ND, particularly when loading and unloading the gun. There are those who say "training" prevents that. NDs occur, though, even among trained troops and police. Old pros have NDs. I remember... well, I won't tell war stories, but it happens to guys who think they are gunslingers.
It cannot happen to me if I don't carry a round in the chamber.