A pocket gun isn't for everyone and won't be useful in every circumstance. But life is rarely all or nothing.
A pocket gun can save your life (or that of another person) [where allowed by law]. Even if you are ambushed and stabbed (or shot) the fight isn't necessarily over and a pocket gun may still be accessible to you to fight back or to prevent a coup de grace. And, if you are responding to aid another person - you will probably have time to draw a pocket gun, no? And having a knife is a good idea, but is it much slower to draw a pistol from a pocket than a knife (since not everyone has a knife on their hip).
And, even if a knife wielding assailant charges you from 20 feet and you can't draw your pocket pistol in time, maybe you can try and evade the assailant, or throw him or her, or sacrifice your arm or leg to a slash long enough to draw (in which case you may be able to stop the assault one way or another). How many knife-wielding assailants will stand and fight against a gun?
Others have already made the point that people often walk with their hands in their pockets, and while I don't like to compromise my hands, if I am in a dark and lonely place, my hand will generally be in my pocket (and not for pocket pool).
