I was responding to the "having the ability for penetration is sometimes necessary" argument. It was stated by someone else... I don't think penetration is very important at all (unless you're using FMJ or ridiculously hot loads... neither of which I would imagine being in a defensive handgun. This is again assuming that a carry weapon is intended as a defensive measure only)
What I was really trying to get at was that different rounds/calibers etc penetrate different materials better than others. Some penetrate people a little better than you might hope, but not necessarily things such as engine blocks or bricks as well... while others are the other way around, and some go through everything and anything, while some might have trouble getting through a folded newspaper. If you're worried about penetration, make sure you check to see what you're worried about getting through, be it a car door or if you just don't want it to go through anything more than the front half of the first person it gets near.
Saying the 5.7mm will over-penetrate in people is somewhat misleading when the "omg it shoots through everything like a laser" penetration characteristic is generally only quoted as that of the ap round... the penetration of which is rated in steel and body armor, not people themselves. The round was originally developed to limit overpenetration in people. Small, high velocity rounds will stop better inside meat/flesh than they will in something like kevlar, ceramics or composite meshes used in body armors. I'm not sure where exactly the line is drawn, but the higher the velocity and the thinner the profile of the round, the less likely it is to be significantly impacted by body armor, but I would assume that even rounds that were designed as AP are designed to expand once they're inside someone's ribcage... as long as it's an anti-personnel round at least.
Long story short... if you can't hold the gun and you're not comfortable with it, you should definitely NOT have it as a carry weapon. I like the five seven, but I have no crevasse on my body that would conceal it's grip personally.