It's not the most common thing, but it does happen. Also, it's not a bad idea to carry a spare mag for clearing malfunction so you don't have to use the same mag that may have caused an issue. For me, I train to shoot the threat to the ground. It will likely be a very fast set of 4 to 6 rounds. Depending if they go down or not, it may be more. The same may apply for any other threats as well. Now, when I needed a handgun for defense, once I fired one round and the next time none.
Likely? No, but it can't hurt and you just may be that 5% that needs more than 10 rounds. Look at that Carl's Jr episode where the two cops shot that guy. Both fired 5 rounds quickly. There was another instance, which I think was shown on "I Survived", where the woman fired an entire mag (though it was 22, but I imagine it would have been the same with 45) at one of the three assailants. He died outside eventually, and I believe the others ran him over as well, but that may have been a different event.
A firefight may very well be a defensive situation. There's that video where Deputy Dinkheller fires an entire mag from his Beretta 92 and then gets killed when he runs dry and fails to reload. I know these are cops, but it could happen to anyone… There's another where an officer pulled up on some property and the owner went after him with an AR. The officer had no reloads, I believe he was wounded but not mortally, and took the bad guy down with his last round as he was coming around the car.
There was another where a guy went to his friend's office for drinks. There was a few of them meeting. One guy robbed them and took them up to the office at gunpoint. The friend pulled his compact 1911 and emptied it at the guy, while the bad guy was emptying his handgun as well. Both were hit, the good guy in both hands. He reloaded his gun and the bad guy fled. I know it was only 7 rounds, but it was still a situation where an extra mag was needed.
Again, it's not all that common, but it certainly does happen. If you're already taking the steps necessary to defend yourself should the need arise, it makes sense to cary an extra mag. I carry one extra mag at all times and two at work.