The issue has been raised about extending a public thread privately by using PMs or going off site and/or offline to continue the correspondence.
I think eveyone knows I am very much against a responder offering such contact. I am not against the contact itself, but believe it should be the asker's choice and not the responder's offer.
One of the purposes of keeping discussions public on a site like this is to get a variety of responses or input on a question. Another purpose is to have peer review to make sure answers are accurate. Taking the discussion private removes both of those advantages.
However, I do recognize that there are occasions where it does make sense to take a discussion private. Generally this occurs when the information being shared is private and posting it publicly might cause problems or embarrassment to the asker.
My feeling here is that we open this for discussion which may result in a clarification of the guidelines. But if anyone sees an offer for private extension of a thread that the Report Inappropriate Post link be used and let the mods decide whether the post oversteps the guidelines or not.