Well, I never really gave it too much thought, but when you think of how much money, time, and effort is spent by people purchasing "that perfect firearm/ammo" to keep yourself alive, what's the harm in employing a passive protective measure as well, such as a ballistic vest? Now that I think about it, in a strange way it's almost silly NOT to wear one. After all, it's protective gear that's not ever going to injure yourself or someone else. Your 5 year old kid isn't going to accidentally kill your 3 year old kid with it. It costs less than most good handguns or rifles, it doesn't take practice to become proficient with, or have a continuing cost of use associated with it.
It is certainly not the end-all, it can be uncomfortable, and WON'T make the wearer Superman, but when the chips are down, don't you want all the advantages you can employ to make sure you survive the encounter? That's why we all spend our hard earned money buying firearms that 99.9% of us will never actually use in defense either. Many, many people have stayed alive solely because of ballistic armor. I just don't see the harm in it.
Afrer all, how many police officers would go out on the beat without one? Well, smart officers I mean. I know Waverunner wouldn't! [:)]