While I think ESR is a bit high and mighty sometimes, I think this exerpt from his guide to asking smart questions is spot on:

Don't ask people to reply by private email
Hackers believe solving problems should be a public, transparent process during which a first try at an answer can and should be corrected if someone more knowledgeable notices that it is incomplete or incorrect. Also, they get some of their reward for being respondents from being seen to be competent and knowledgeable by their peers.

When you ask for a private reply, you are disrupting both the process and the reward. Don't do this. It's the respondent's choice whether to reply privately — and if he does, it's usually because he thinks the question is too ill-formed or obvious to be interesting to others.

There is one limited exception to this rule. If you think the question is such that you are likely to get a lot of answers that are all pretty similar, then the magic words are "email me and I'll summarize the answers for the group". It is courteous to try and save the mailing list or newsgroup a flood of substantially identical postings — but you have to keep the promise to summarize.

-Blake