As I understand it, the original DIR forum was created, at UP's instigation, as a place where people interested in the system could come and ask questions about it, without being piled on by people either giving invalid answers, or criticizing the poster for being interested in the first place. It actually worked reasonably well for a long time, thanks to some fairly draconian moderation. (UP, for example, would remove trip reports or other "DIR" topics, as they weren't questions about the "how to" of the system.)
With some major changes in moderator personnel a few years ago, that level of moderation fell off, and the forum began to change form. This led to the discussions that broke the DIR forum into two pieces -- one, as Jeff says, permitting discussion, and the other theoretically not doing so. I don't think there has been much issue about the threads in the Practitioner's forum, but basically nobody posts there.
It is difficult to see how to moderate the open forum, as non-DIR answers are permitted, as is discussion and argument. So what do you throw out? I agree with Brian, that the noise can overwhelm the valuable content there, and the only way a reader knows what's DIR and what's not is if DIR folks post to identify the non-DIR answers as such. Short of having a canned response to the OP, identifying the two forums and suggesting a reposting, I don't know what else we can do.
I think, honestly, that some of the discussions in the open forum have been quite valuable, and I enjoy the input from some very experienced and knowledgeable people who may critique the system in some details. But probably, those discussions could simply take place in the general scuba forums. The original questions that prompt them, however, probably wouldn't get posted there . . .