I reckon you might as well leave your car and house unlocked (I don't follow my own theory though I must admit).......if a criminal is worth his salt he's going to assume that the house is locked anyway and that didn't stop him.
Leaving it open just avoids the barreled lock or smashed window.
Then again it does give the unshaven, unskilled scrotters a better chance of having it away with some valuables I guess.