I generally obscure mine for fraud purposes - license plate cloning. It's one thing if you see my truck parked in the driveway, obviously the plate's readily visible... but one can do a google image search for "white pickup" and possibly come across mine.
So some guy in teh state or country with a truck like mine makes a plate similar enough that a camera can't tell a difference between it and mine, then goes off blowing tollbooths, pump-and-run, or any other sort of crime... i'd be the one getting the phone calls about it, if not worse.
Or you could play a sick joke on someone and phone it in as stolen. Here's the vehicle description, here's the plate. Especially if you're posting it for-sale.
A couple of quick swipes with a clone or smudge tool, or just frame the shot so the plates are obscured... not too hard to do, could save a lot of headaches down the road.