Hell with jamming, what if you have something in your hands, or you have something on one of your hands that prevents you from getting a firm grip and causes your hand to slip. What if in the moment of panic your fine motor skills quit working and you are unable to to successfully chamber a round?
I used to carry when I wasn't out and about in a Serpa holster. I had practiced drawing my gun from that holster thousands of times, it was second nature to me. One day while fishing I got excited about the prospect of shooting something (what I won't go into detail of), but in the excitement everything I knew failed on me. I about ripped my pants off trying to draw my gun from the holster without hitting the release on the holster. I wasn't even scared, just excited.
In pistol classes when I used to carry a 1911, under stress or excitement I either missed or forgot to disengage the thumb safety on my 1911 several times over the course of a couple of years as well. Keep in mind that I carried a 1911 for years and sweeping the safety was second nature for me, hell it still is, I sweep the non existent safety on my Glocks when I draw them to this day.
Now I won't carry in a holster that requires anything more than pulling the gun out, and I won't carry a gun that has a safety. No way in hell would I carry one without a round in the chamber.