There's no question that carrying with your gun ready to fire is the fastest and most combat ready way you can possibly carry a gun, I won't argue that. Even if there were no speed differences, carrying with one chambered is still better for the fact that you can get a nosedive by chambering a round.
With one already in the pipe, there's a very high percentage it's going to go boom when you pull the trigger.
My ONLY beef is with those who tell someone they are essentially wasting their time by carrying a gun without one in the chamber. Most of the people saying that have 0 experience with a life and death situation and in fact have no idea how they themselves will react, perhaps they would be better of with a gun that isn't loaded.
There are way too many unknowns and not everyone is the same... I don't care how you carry, just practice that way and I'd go so far as to say don't alternate.