Not giving private support has been a policy on phpBB.com since I joined over three years ago. There is, in fact, a rule against contacting team members for private support:
Support is offered only via these forums and #phpbb on irc.freenode.net. Do not contact team members privately (via any method) to ask for support. Users found to be contacting team members asking for support will be warned.
While this may sound a little stern (it is by far the most warned offense here at phpBB.com), the reasoning for this is actually quite true.
Most people who contact team members for private support do so because they figure they can get a faster response and that doing so will take very little time for the team member to answer because they “know their stuff”. Now, either of these cases may be true if only one user is doing it. Let’s now consider the scenario where multiple users a day (maybe even 5-10 or more) have this reasoning and choose to do so. That’s 5-10 PMs that must be answered or handled per the rule above. Now repeat that again for 50+ team members. Assuming each PM takes 2 minutes to deal with (this is a very sparse estimate), that’s 8 hours 20 minutes of time spent dealing with these PMs instead of helping other users (not to mention the time the Moderator Team spends issuing warnings).
Just consider for a moment what we could do with that time. Most users PM team members because they think their response will be quicker — in fact, by doing so, you are not only slowing down the response you will receive, but the response others will as well!
The reasoning, then, is quite simple: because private support will take time away from you, the end user. Answering questions takes time. Add on to that the fact that a PM helps only one person, and you have a lose-lose situation. Most of us would much rather take the time to answer topics in the support forum that could potentially benefit hundreds of users than spend equal (or more) time answering PMs that will help only one. 
So, remember: next time you go to send a PM to try to get a quicker answer, stop and think for a second: What will be the side-effects of this? Aside from being warned, it takes time away from helping you, the user. The warning isn’t since we’re uptight and don’t have a sense of humour (believe me, we do) — it’s because such PMs take away from our time that could otherwise be spent helping you.
Thanks!
Don’t forget the other side of the story. It certainly isn’t a good thing from the POV of the people answering questions, but what about the people asking the question? Is it really such a great idea to contact one single individual privately? Of course not, because in that case you are depending on that one person actually knowing the answer and for that person to actually come to your question in some timely fashion. If you post on the board, the chance that someone who also knows the answer sees your question before your designated guru gets to, is quite large, actually. So, the rule also benefits the people that ask questions.
Any forum really should have a policy of “ask questions on the board, answer question on the board”. Some users will also add “Please reply by email because I don’t come to this board very often” at the end of their post. No. If the board is worth coming to to ask the question, it is worth coming back to, to read the answer. Besides, usually you can get an email subscription, so it doesn’t mean you have to monitor the thread for answers, you’ll get a notification when there is one.
The other side of the story is that users need to understand when personal communication is better. Sometimes, threads get several subsequent posts, alternating between A and user B, that are really of no interest to anyone but user A and user B. In that case, they should have taken the conversation to PM or email.