It all depends where you are though.
I mean, here in South-Western Sydney, lots of things get stolen and whatnot.
One time my friend took my magnetic P-plate when I was in a car park queue, then stuck it on the car in front. LOL.
She grabbed it back when the car in front started driving off. >_>
Ooh, I know someone who has an expensive Merc convertible, on P's, and the brake light got knocked out of place, after parking it somewhere.
Considering I share my car, I get into the habit of taking the plates of since I heard if a non P plater (i.e dad) gets caught with P plates on, he gets fined and loses like 4 demerit points = dead me.