i think the idea of double locking doors might be this: if someone wants to STEAL your car, brakes a window, but sees that he cant unlock it, it might not be a good idea to drive off with it because it willb e a pain in the booty.... i dont know, maybe not?
or if a crook wanted to keep it quiet not breaking any windows, if the windows are down a little bit, trying to put his hand down to unlock or using a coat hanger..
but seriously... if a crook wants your car, they can get it... its not too hard.