I can think of a couple of different reasons for not chambering a round.
If you have only a single handgun that you'll be using either for OC or CCW then one of these may be moot. Having just one gun would mean that you should be practiced to death and totally familiar with that firearm, to the point of 'no thinking, only acting on instinct' in any defensive situation.
But I think what the OP is not saying is that he relies on situational awareness. Consider the person who's gone decades without carrying, and staying alive in the process. Those people generally avoid edgy situations and feel they'll have plenty of time to prepare/react should something go awry. I tend to fall into that category. I'm just happy to have a gun on board, chambered or not. If I've gotten myself into a situation where I don't have time to chamber a round then my head's been buried pretty far up my ass for that given moment. Let me add, create DISTANCE and chamber a round.
No need to pounce me for that, just an observation.
Add to that if a guy is open carrying and some punk manages to snatch the firearm. Imagine his dismay as he points your empty chamber at you and makes his demands. To quote Billy Jack, "My right foot is going to kick you on the left side of your head and there's NOTHING that you can do about it" (pp). LOL that's indeed a possibility. Have you ever looked at a cop and told yourself that you could have his gun before he ever knew it was coming? I've done it since I was a kid, and I'm one of the good guys.
But the other angle is if you have several different types of handguns. DA revolvers are point and squeeze, no thought needed. But autos bring their own equations, depending on model. Safety? Single action? DA/SA? Up until now I never cared to own or consider an auto as a primary protection unit. I've always trusted the simplicity and reliability of a DA revolver. I've always been concerned about the reliability of an auto. I was never concerned about a chambered round in a revolver simply because that hammer was resting down (no spring fatigue as well). A DA is a total no-brainer in an adrenaline filled gut reaction panic situation (for me or my gal).
But when I start carrying with a hammer back I start considering the possibilities. What if that safety is inadvertently left in the wrong position? Do I TRUST the safety? What if the gun takes a blow? If you're someone who's life is filled with relatively safe surroundings all of the time then you may just opt to keep the pipe dry while keeping your eyes open. In those cases, better to have a gun than not, which is how a lot of people feel. Protection is a split second snap of the chamber away.
But if you DO have several different types of handguns, think of how they differ. We have a variety of guns in every sort of action. The shotgun is a home protection pump with a chambered round with safety on. I have to visually remind myself of the safety. Big game rifles are standard bolt action. We have two Beretta 92xx handguns that have their unique hammer down safety features. That model is a DA/SA. Once the hammer is down and safe with a round in the chamber I'll take the safety off so it's effectively a revolver action on the first round. Point and shoot, no thought of a safety. Great OC gun but too big for a comfy CCW.
But we also have two military 1911's. With those I have to be thinking about squeezing the handle and releasing the safety while that hammer is always locked back. No big if you're prepared for it, but what if I've been carrying the 92's for weeks before that? I'll be squeezing without results. I probably won't be carrying the 1911's. We also have a Colt Mustang Pocketlite .380 which is SA only. I don't like that action for carry. Will I remember the safety? Can I TRUST it? Instead it's hammer down and safety off with the reminder that I need to pull the hammer back, and on that unit it's very easy to do if needed.
So for some the 'rack one in the chamber' mentality may be simpler than thinking about different safeties or actions. The biggest drawback to that in my mind is the SOUND it makes if you're trying to be stealth. Pulling a hammer back can be fairly innocuous in a tense situation.
The handguns we have were inherited as a result of a passing family member so they're not my first choice for carry. I prefer a true DA/SA that's point and shoot ready without any sort of safety. If I end up with an all around favorite such as a Glock 17 or some type of Sig then I may train into it to the point that I don't care about a hammer-back-safety-on situation. I still lack trust for autos but am quickly coming up to speed.
So chamber empty is great for the guy or gal who's just happy to have a gun around where in the past they did not. I can understand why they'd prefer it. Situational awareness should ALWAYS be your primary form of protection. But if you know your gun and can act instinctively in every situation because you are well trained and practiced, keep that extra round in the chamber. Racking a round can be extremely difficult with an overtly sweaty pair of hands. Soak your hands in water and try it out.
Besides that obvious speed advantage, that extra round MAY just come in handy...