Credit card signatures: Useless?

SpaceAdmiral writes "Everyone should remember John Hargrave's classic Credit Card Prank on Zug. He tried signing fake names on his credit card receipt, and no one seemed to care. But that's nothing compared to The Credit Card Prank, Part 2. Can he draw obscene pictures instead of signing his credit card? Yes, it turns out. Is there any way of getting your signature checked? . . . Yes, it turns out. But you have to do an awful lot."