^Hell, it's why democracy with rule of law triumphed over other forms of government, at least so far as material culture is concerned.
What I find funny about Hawking's pronouncement is the assumption that we could hide Earth from an extraterrestrial intelligence capable of hurting us. If they're looking, they would likely be able to image Earth and determine its size and chemical composition, whether we were here or not, or make noise or not. If Earth-like planets are important, they will be looking, either remotely like we do, or if resources like ours are important enough to seize, with probes.
By broadcasting our presence, we would be doing a favor to societies which generally try not to screw with other intelligences, who might then deprioritize us on the list of colonizable planets; but we wouldn't be saving ourselves from amoral aliens, because if they want Earth, they'll come and get it, because our little blue ball is not at all invisible to anyone who cares enough to look.