Basically, the claim that any criticism levelled at the scheme is premature because the critics have not tried the scheme. The implication is once the critic have tried the scheme s/he will change his/her mind. It basically takes this form
A: Acme XYZ is a scam because of ____, _____, and _____.
B: But you don't know Acme XYZ. How could you when you're not a member? Join us.The reply sounds very sensical, until you realize one thing: It never addressed your point: "Acme XYZ is a scam". It is completely irrelevant. It is a red herring. It neither disproves your premise, nor does it prove a counter premise.
The argument is non-sensical, and here's a very appropriate reply quip for such idiocy:
"So you have to eat shit to know not to eat it, huh?"
(Thanks to justicealwayslate on Facebook)
There are plenty of other quips, like "oh, so cops have to be criminals first to arrest criminals, huh?" or "do I have to shoot myself to know it's a bad idea?" or "Do morticians have to die to be a mortician?" But you get the idea. It's ridiculous.