I don't necessarily disagree (see my original post) but the problem is that, to use the analogy you make, that if it's a forum for the discussion of problems in the (legal) system, and if you only had actual lawyers, and no police, or forensic scientists, and weren't sure if you wanted judges or not, and were debating whether a law Lord was or wasn't a good person to have there, ..... you might end up discussing the technical details of the law, but miss some of the bigger picture.
Or, back to the aviation world, pilots can discuss until they are blue in the face why someone did or didn't follow the manual-ordained procedure, and whether it was the right decision or not. But if there's no-one involved who's seen the manual development process, and can perhaps explain why it might be so apparently confusing or misleading, then the discussion may end up a little sterile.
And the other forums WILL undoubtedly suffer, even if they are still there; if the 'restricted forum' is seen to be a quieter place for discussion there will be an inevitable tendency for issues to be discussed there which would otherwise be discussed in, say, Tech Log or Safety CRM & QA. That will tend to lower the quality of discussion in the other forums, and you may find a vicious/virtuous circle develop (depending on which forums you read!).
For my own part, I find that observing the (often wide) range of pilot community opinions on aircraft operations to be a valuable background to my 'real job'; you could be building what you think is the best tool in the world, but if the toolmaker doesn't know much about how the tool is used .... so I would find it something of a loss personally. But I do understand the desire to keep discussions in a controlled environment, and it would be very hard to devise an 'admissions policy' that wasn't based on something simple like license status.