I'm a pessimist.
I carry a 10mm Glock 29 and a spare mag. I keep its big brother, a Glock 20, in my nightstand... I would feel OK with the G29, but I don't want to risk forgetting to bring it in there at bedtime.
My wife is such a pessimist she prefers revolvers, carrying a J-frame and keeping a .357 Magnum in her nightstand...
That's not pessimistic, you say? Well, it's an 8-shot revolver.
So, when it comes to shotguns for home defense -- meaning that the handguns have proven inadequate, or there are apparently enough visitors that we bypass them entirely -- mine is a Saiga-12 with nine rounds of #00 buck, and my wife has a Mossberg 500 with five 1-oz. slugs.
I'm not arguing that a 20-gauge is inadequate. I'm simply saying that, in the awful event that we need to kill, we prefer overkill.