Off or On?

This is something I have been passively thinking about for years and today is the day I finally got around to finding an answer. I guess I should start by telling everyone the question before I give you the answer. There are two households in my neighborhood who leave their porch lights on every night and I've always wondered if that is something they do for safety or are they expecting late guests,or do they want to insure if someone breaks in they can see what they're doing so as not to injure themselves then turn around and sue the homeowner. The bulk or experts say "no" to leaving porch lights on unless it provides comfort to a nervous occupant,they say that if someone was going to break in they would scout your house beforehand and know that you routinely left your lights on. What they suggest instead are motion sensors which often startle would be intruders and can serve to alert neighbors who notice a light coming on. They all agree on one thing with certainty and that is NOT leaving porch lights on if you are on vacation.