A simple equation. You are alone with your family and you encounter a gang of hoodlums. If you are unarmed it doesn't matter if they are or not -- you have no power against the mob. You lose. If you are armed, even if they are armed you represent a hard target. If there are easier targets they will go elsewhere. You win.
As quoted in the Tribune today:
John Carolan said he was sitting on his porch in the thick humid darkness just before midnight Tuesday when three or four young men, one with a knife and one with a machete, stopped in front of his fence and pointed to the generator humming in his front yard. "We want that generator," one said. "I fired a couple of rounds over their heads with a .357 magnum," Carolan recounted Wednesday. "They scattered. You've heard of law west of the Pecos. This is law west of Canal Street."
You are responsible for your own protection in cases like this, and in general.
Charon