There is a saying:

"An armed society, is a polite (safe) society."

If Jack knows there is a strong possiblity that Jill MAY be carrying, the liklihood that Jack will attempt to cause her physical or mental harm or otherwise impose his will upon her; is far less likely that if Jack is 99% sure she is only carrying lipstick in her purse.

Or if Jack thinks that if he goes in to rob the 7-11, there MAY be 5 people in there that MAY armed. He is not going take his chances robbing that Habib (Simpsons reference).

I also bet that those drivers that flip people off would think twice if they thought the other driver being armed was a probability. Not that it is Okay to shoot someone over that, but it has happened. I bet the dead guy really regrets flipping off that guy that cut him off without knowing it.