There are many not to uncommon scenarios between the single assailant self defense and the mad man or men on a rampage, presented by the OP.
As in the car load of gang bangers who gunned down a little girl because she was wearing a red dress, with fifteen onlookers doing nothing or the group of thugs in Seattle who were gang rapping men.
That said the likely hood that I will ever have need of my weapon is unlikely.
Yet I carry in the event that I do need it.
Those that presume that one or two shoots should get the job done, maybe right or may have never had to make that shot while being shot at, I don't know.
Unlike other greater men I accept that I fall short of being the ultimate warrior.
So I train & practice as best I can, and carry whatever may help stack the odds in my favor.
Thats fiber optic night sights though I may never take a sighted shot in SD.
A laser though I may never have time to turn it on.
A spare mag though though I may never need more than a few rounds because I can not always pick my battles or the conditions that they may occur.