Ammo capacity is a good thing. Its very comforting to have. It's like fuel, as the pilot's saying goes, "you can't have too much fuel, unless you're on fire!"
Trouble is, many people, if they have the capacity, will USE it, when they don't need to. The military and the police have a degree of legal immunity. You and I, DON"T.
We are legally responsible for every round we fire.
The most common "training method" today is action movies and TV. It is BAD TRAINING, but its free 24/7 on tv, and it does have an effect on people.
Action stars shoot a lot, provide their own cover fire. They also don't face legal consequences (and they're not even firing live ammo).
As a private citizen, one should, as Mas Ayoob put it, "shoot only in gravest extreme". That does NOT mean shooting to keep the bad guy's head(s) down like one sees on TV. There are rare situations where it might be the right tactic, but its not the right "attitude" for personal legal self defense.
SO, having 15 rnds doesn't, or rather shouldn't mean you have to USE 15 rounds.
The undertrained (and this all too often includes the police) who have a lot of rounds onboard tend to use a lot of rounds. Sometimes, they even hit the intended target. Sometimes.
Those with fewer are often more prudent about ammo expenditure.
When it comes to stopping a determined attack, only hits matter. Afterwards, to the law, every shot matters.
Home invasion by a group (4-5)? yes, our worst nightmare. But, are you facing a banzai charge to the death? Or are you more likely to face people who lose their enthusiasm for attack after you shoot holes in the first one or two?? if you even have to shoot??
Every time this kind of thing comes up, I am reminded of a standoff scene from a comedy movie. One good guy, facing about a dozen bad guys...
Head bad guy says "what you gonna do?? you think you can shoot all of us??"
Good guy: "No, but I can shoot YOU!"
the head bad guy pauses, then says "oh, yeah, I always forget that part!"
I realize every situation is different, but from listening to the news all my life, it seems to me that most of the time, if a bad guy or two get shot, the rest usually find a good reason to be somewhere else, as fast as possible.
Hope for the best, plan for the worst and you are seldom disappointed. I'd be good with a revolver that I was good with, over a hi cap semi that I couldn't hit the broad side of a barn with. Equally good with both? then having more rounds available can't hurt. Using them without need, might.