I read this a long time ago and agreed with it completely but had lost the URL. The document Why Ask Questions in Public? does a good job of explaining why one should shouldn't reply off-list by default and directly engage helpful list members privately.
Allow me to quote a bit here...
I, like many other people with technical expertise in some topic, regularly read various Usenet newsgroups and mailing lists and try to answer questions for which I know the answer. However, I am also extremely busy with a large number of projects, often including improving the very software that people are asking questions about, and I've found that taking some time to help other people sometimes has the regrettable tendency to add to my total level of pending work. This is sometimes hard for me to deal with, and I'd like to ask for your help in solving this problem.
The problem generally takes the following form: Someone posts a question about something I know something about. I respond in the newsgroup or on the mailing list with some suggestions or possibly some questions. In response, the person mails me directly (rather than responding in the newsgroup or mailing list) with the answers and with other questions. Occasionally the problem takes another, related form: I post about a topic, someone who has a question about that topic reads my posts and thinks I sound knowledgeable, and they send me e-mail asking me questions.
I understand why people do this. Often they view newsgroups and mailing lists as giant, impersonal places and want to get out of them as soon as they can, and as soon as they find someone who can answer their questions, they latch on to that person and want to interact with them directly. I'm sure that they don't understand that this behavior causes problems for the person of whom they're asking questions.
Go read the rest to see how it concludes. Tell others who seem to habitually get this wrong.
It always surprises me that people who've been on-line for years seem to somehow not get the importance of keeping some discussions in the open. I don't know why. Even after repeated prodding, they seem to lapse into their old habits.
What's even worse for me is that we seem to have some of the worst offenders at work. :-(
Am I alone here or do others see this happening too often? Do you try to do anything about it? Have you found an effective tactic? I haven't.