Always being the "devils advocate" (my wife hates me for this), there are a few problems with doing it that way.
The one that comes to mind first is that there are people who are not considered modders (and have no interest in modding) that might be able to contribute to the process if they are following the WIP and the open exchange of ideas in the posts. There are also folks that are just getting involved (or considering getting involved) because of the exchange of ideas that they see in the open forum.
The other thing I see is that there really is already a method for private communication between modders, the PM. While it's not quite as flexible as a private forum, it still serves the purpose of passing information back and forth.
The whole idea of a forum is the free exchange of ideas and opinions. IMHO, restricting that freedom may limit the process rather than facilitate it.
We all have disctinctly different personalities and some of us suffer from the "please everyone all the time" syndrome, which can lead to being pressured to release a project before it is ready. We, as consumers of the modders work, should refrain from applying that pressure. And modders should do their utmost to not succumb to any pressure (real or just perceived).
Just my extremely verbose opinion.
