While I'm sure I've commented on this before, I was reminded of the capacity debate again today. A girl bought a Five Seven and commented that the "other guy tried to sell me a revolver or something. I just don't feel comfortable with 5 shots."
I told her that 20 rounds could only be useful in very limited situations in which a person had found cover. I think I had her regretting her decision until I said with a grin, "But I have to admit they're cool; I even have one on layaway." It made her feel better, and she left happy with her purchase, though I think I may have given her something to think about, too.
On one side, the capacity debate seems clear-cut: no one ever lost a gun fight by having too much ammo. On the other, things are a little fuzzier: stopping power, good accuracy, and awareness should mean that a two-barrel derringer is plenty, if you know what you're doing.
A few people start to look at the different goals of different weapons. If you're looking for a self-defense weapon, the derringer side is right: you won't need more than a couple shots. On the other hand, in a survivalist's view, a high capacity means that you can hold a lot more rounds in a more ready, contained, packable package. With one mag in the Five Seven, two in a mag pouch, and one in the chamber, you are carrying 61 rounds without much effort. That makes it easier to pack more equipment.
As far as I'm concerned, the defensive shooter needs little more than a .45 or .357 derringer, but that's no excuse to limit yourself to revolvers and below for defense. Besides, you're more likely to actually get good with something you don't mind reloading enough to practice plenty.