Some people actually like to get those custom cardbacks. I know in the case of a friend of mine, he's gotten several figures on custom cardbacks. For instance, he got a Rystall, from the "Jabba's Dancers" cinma scene like that. He knew it was a custom, and he bought it to get autographed by the actress who played the role. It actually looks a bit better than the Cinema Scene I got signed by the same actress.
Of course, that doesn't really apply in this instance. First, Sir Alec Guiness wasn't a prolific autograph signer, and the man passed away a couple of years ago.