Wrong! Why even bother carrying a weapon if you aren't completely prepared to use it??? That was one of the first rules I learned in my concealed-carry sense, and frankly, it's make perfect sense. If I'm in a situation where I truly need to use the weapon, I can't be wasting time chambering it.
Maybe a little thing like safety. Nothing like blowing your wife's brains out to remind you that guns are dangerous. Or maybe you will luck out and only shoot yourself. If one would bother to think before spouting, would you rather simply pull your gun out and shoot a potential target; or give him the opportunity to reappraise his situation by showing the gun, and then chambering a shell? The time difference between chambering an auto, and drawing one is neglible.