My thoughts on the matter.
If you have to reach for your gun AFTER you are approached and have a gun stuck in your face, you probably shouldn't be carrying one. If you are simply an idiot and driving around in dark alleys with your windows down and door unlocked and are paying more attention to your cell phone than your mirrors, a gun isn't going to make you any safer.
A gun is a means to end unlawful violence as a last resort. There is no substitute for situational awareness.
I carry my USP .40 at all times. Civic duty and all that stuff.
That said I have never felt threatened and I have never felt I "needed" it. Just another thing I pick up along with my keys, wallet, and phone.
But to answer the OP, because in America in 2005, there are still drug dealers, rapists, thieves, car jackers, murderers, child molesters, arsonists, stalkers, meth heads, etc.
The unprovoked and unlawful transgression on another?s life, liberty, or property is grounds for removal from society in my book. Anyone willing to violate these three principals has shown that they cannot coexist in a peaceful civilized society and respect fellow human beings at the most fundamental level. The only time the taking of life, liberty, or property from another person is lawful and acceptable is when society deems it necessary in the punishment of criminals in order to preserve the social order that protects lawful life, liberty, and property.