For an analogy, look at the skull and bones society at Yale University. There are deep, dark secrets tha only instructors who have passed the initiation rites and gone through the ordeal of terror may know. For example, we know the steps of the ceremony we are required to folllow on the anniversary of the death of Jacques Cousteau, and those secrets must never be revealed. We must have a forum where we can allude to these secrets without fear of that they will be revealed to the infidels.
But seriously folks, there is a theory behind it that does not actually play out all that much in actual practice, and much of what goes on in that forum can and does get discussed (to exhaustion) in other forums. Most of the I2I material could be in a public forum easily.
The best parts, and the parts that make such a forum a positive part of the overall scheme of things, are when instructors ask for advice on how to handle tricky instructional problems. If there were no Instructor to Instructor forum, an instructor might hesitate about asking a question like that, for fear that the general public might fear that he or she were an inferior instructor instead of being the high class person who wants to refine his or her technique in a community of colleagues.