Perhaps security pester aircrew just as much as civilians because they can't tell the difference between a real crew and someone who just bought the uniform at a fancy dress store. If anyone with stripes on their arm was just waved through then all the bad guys would have stripes on their arms. Remember Frank Abagnale jr. Once I recall there was a panic in Rome when someone stole crew uniforms from a hotel, though in the end they were never used in anger.