Public discourse is important.
And when I publicly challenge you to provide such evidence and you give me information in private email but do not give me permission to rebroadcast that information... I am angered. This sort of back channel "persuasion" does not get to the heart of the problem. Because well, I am not the heart of the problem. I may be your PR problem..but that's not the real problem. Convincing me of the righteousness of your opinion doesn't help solve the underlying problem. If your hurt feelings are substantiated by the evidence, having that information stop with me..and preventing me using that evidence to start _fixing_ the actual problem is not in your best interests. When such back channel information sharing is occurs I view it as manipulative.
I don't have a problem if someone takes a discussion with me private because they feel they need the space that a private email discussion provides to focus without sidebar comments or commentary. But when the discussion is over I fully expect to be able to republish a private conversation in its entirety. It's only through publicly disclosed discourse that people can be held accountable for what they say, including myself.