I like to hide the number plate in an advert. My thinking is (as above), anyone serious will ask for it to do their checks. Trust your gut reaction if you're happy to oblige them...
Secondly, having your number plate cloned is serious agro. Drive offs at petrol stations, speeding tickets and much worse will be coming your way if a member of the underclass decides to help himself. Sure, your reg is freely on display on the vehicle but why make it easier for a potential crim to get the details without even leaving the comfort of his sofa?!