And there's no need to tell the judges who the sceptic is anyway.
They would know. Firstly, Demarco (assuming she'd be a judge again) would know if she's ever heard of this person before. If not, she'd naturally assume a ring-in. It seems previous contestants were aware of each other. Mitchell does magazine and radio, Jason (believe it or not) was Psychic of the Year 2007 (?), Charmaine and Ezio are friends, She and Rayleen both have websites. Not sure about Amanda because she was supposedly a non-professional but she clearly has a history. All are members of the Association.
Besides the judge, the public know if they've ever heard of a psychic before so it would be hard to claim you're showing "the best" yet not have anyone who's known publicly. Take a look at the earliest comments on The One website. Many are from people expressing support for their own favourite psychic - someone they knew before the show aired.
Add to this the likelihood that there's probably very few Australian skeptics who are doing enough cold reading to compete at this level (although the demonstrations were pitiful so I could be wrong here) and they'd likely be known as skeptics.
Anyway, all the psychics would KNOW - they're psychic!
As for the entertainment value of debunking, I have to disagree. On any other "reality" show, we'd see contestants being humiliated for "our enjoyment". Whether you or I agree with the practice doesn't change the fact that, ordinarily, Channel Seven thinks humiliating contestants is good for ratings. Yet we didn't see humiliation of the psychics (other than there own failure).
I can't explain it, maybe Richard could, but Channel Seven seem to have a vested interest in protecting the reputations of people who claim to be psychics.