Two different questions...
Think of its potential use.
Lots. Plinking, short range hunting, pest control, sport, and even home defense.
Then wonder what it would do in a self-defense scenario that couldn't be done better by a 12 gauge shotgun.
Not much.
And that is becuase a self defense/home defense scenario is radically different from SWAT entry team assaults, police use, or hollywood fantasies.
Large magazine capacity and range are the advantages over the shotgun. And for a civilian in self defense, these are rather moot points. Providing you own cover fire is a great thing to watch in the movies, but in real life, you are responsible for every round you fire, with no govermental agency behind you to pay your legal costs.
Range is moot, because, face it, how far away is the threat? 60-80yd (and longer) are doable with a pistol caliber carbine, but in a self defense situation, are you going to be justified shooting someone that far away? I just don't see that as likely. Unlikely to actually know they are a deadly threat (and so justifying deadly force response) until they are much closer, where the shotgun is useful tool.
And if you did shoot closer, and now they are running away, even though you might be able to hit them at 50yds plus, you aren't justified in shooting.
No matter how much you might want to.
Anything works for across the room, or the barn, but for self defense, the range of a rifle usually isn't a big factor.
The pump 12ga is an American cultural icon. And it works. And they can be made as handy as a pistol caliber carbine. They just kick more and hold fewer rounds. I think the effectiveness is pretty obvious.
If your plan is to blaze away and hold 'em off until the cavalry arrives, the larger capacity of the pistol caliber carbine is a useful thing. Other than that, I don't see a benefit over the shotgun. If you are in a place where blazing away isn't a good idea, (and who isn't?) the 12 ga makes better sense, to me.