as for the rule against, they can just break a window to get in and unlock the door that way! besides, if the car's all mangled up, the door might not open whether it is locked or unlocked.

but i lock it when i'm in unfamiliar places or by myself or when i get out to get gas. this is so that people don't jump in the car or grab my purse or things off the seat or something.

note that the purse scenario actually happened to a friend of mine at a gas station. she was pumping gas on the driver's side. some guy opened up the passenger side, grabbed her purse, and was gone. what really scared her was her little boy was in the back seat so if he'd stolen the car, her boy was in there.