For a off duty cop or armed citizen to need a reload is not very common. Most gun fights are 2 to 3 shots.
BUT.... there are enough where a reload is a life saver, so it's a chance no smart person needs to take.
It's more difficult to carry a revolver reload, but a spare magazine for an auto is easy.
Too many people carry a CCW and no spare ammo, and that's a possibly fatal decision you don't need to make.
It's easy enough to leave the house with no gun when "Just running to the store for some milk" and leave the gun, much less a reload for it.
I've been told by a man who KNOWS that the worst feeling in the world is being in a "stop and rob" bad situation and realizing the gun you really, really, need is at home on the dresser.
In the case of a home defense shotgun needing more then 6 rounds is pretty unlikely, given most peoples living arrangements. Loading up the gun with accessories and spare ammo can slow the gun down and actually reduce effectiveness.
But, as above, if you feel that you need more ammo it's your decision.
Just be honest with yourself about what you're most likely to be facing.