Think I brought this up some months back. There've also been a few lengthy discussions on limiting initial posting rights in the past.
I believe a large (and very valid) argument against having members only able to access the intro forum is that we have many, many members who only come to check out the softmod guides, which is fine. But if they're unable to request help in that thread due to its location, they'll start cluttering up the intro forum (which isn't meant for questions) or get fed up and go somewhere else. Neither of these are things we want.
Another was that we have many people who only like to lurk on the site and not post, which is also something we have no problem with. Many people just come for the info, and if they were driven off because of a posting time limit, well, guys like me who eventually want to stick around wouldn't be here.
As far as the thread I mentioned, though the recommendation was approved, think it was one of those things that fell lower down the totem pole as other events developed.
Right now the forum is in the middle of a slight overhaul/upgrade, but when things settle out again might be a good time to discuss this kind of suggestion.