I carry at least one reload any time I leave the house, depending on what I'm doing. While it is always possible that there might be a reason to need more rounds, my main reason is for reliability.
My carry pieces are as follows: a G22, an M&P 9c, a G26, and a 640.
First, if a deadly force situation is encountered, and you're not sure of how many rounds are still in the gun and you also don't know if the immediate threat has gone away or there are more threats; perform a tactical/save-magazine reload.
Secondly, magazines are part of the mechanical complexities of a semi auto pistol. They are probably the weakest link of any semi auto. You can store them for years empty, or for years loaded and they'll function. You cycle them regularly, such as from regular range trips and they'll wear out. I've been using some M&P 17- round mags I acquired used for training and competition. Don't really know at all how many rounds they've had thru them, but the springs are starting to fail. How often do you replace your carry magazine? I carried Glocks for a lot longer, and have a bit of experience with SIGs and 1911s (and Beretta ,22s). I've seen magazine failures in all of them.
Thirdly, just because the statistics suggest that the average civilian use of a firearm is 2-3 rounds fired, what are the stats based on, and what is the high number? In law enforcement for many years, the average number of shots fired by LEOs was about the same as the civilian stars referenced. It was based on NYPD records. Then the NYPD staticians reviewed the data, purged all the suicides and NDs. The average went up to 6 rounds (NYPD still carried revolvers at the time). Current average I believe is 8-10 rounds.
I recall that in the Utah trolley shooting incident a few years ago, the off-duty cop who intervened only had something like 7-8 rounds in his off-duty pistol, and no reload. While his armed intervention prevented a greater tragedy from occurring, he also ran out of ammo before the killer was stopped.
The question of "responsibility to retreat" in relation to carrying a reload is best left to the legal analysts such as Massad Ayoob. There are some good attorneys who specialize in self defense incidents as well as insurance policies for civilians.
