I think Peter summed this up succinctly when he said that if a writer emulates Tolkien for the sake of world-building, he doesn't have "world-builders' disease" as it is currently understood (and described by Brandon) as a problem for novelists.
I even hesitate to use the word "writer" in the above sentence, because when I am world-building for dungeon mastering, I don't consider myself a writer. I consider myself a world-builder, similar (but far less skilled) to Tolkien.
If a would-be novelist emulates Tolkien out of a misguided belief that he must have complete mythology, background, history, or even character history in place before putting pen to paper, he has world-builder's disease.
Tolkien may not have had the "disease" in that it was a problem for Tolkien -- I believe that is Fardawg's primary point here -- but Tolkien did have worldbuilders' disease as understood by the modern novelist -- which is Brandon's and Peter's point.
It's also worth noting, with all due respect to the man, that Tolkien's own opinion of his "true" work was not a realistic assessment of the marketplace. How true of most new authors this is. Yet it's equally true today that a novel within a customized fantasy setting is far more marketable than a rulebook for a customized fantasy setting.
I know, I know, how dare the editors and Fan Dumb define true art. Right?
I agree with these points, Jason.
I've also read fardawg's answer to these points, and I can't help but saying that, having read the Silmarion, it doesn't read like a story. Oh, the early bits (the creation, the music, etc.) and some of the rare interludes, personal tragedies and occasional victories, are interesting, but beyond those, it reads like a history textbook. A dull, boring, and painfully detailed history textbook. If Tolkien was writing it as a story (or, if you prefer, mythology), then apparently his idea of "Writing a Modern Mythology" is identical
in form to "Doing a Lot of Worldbuilding with a Couple of Interesting Stories placed here and there inside of it," even if that wasn't the intended purpose.
Don't you see? The distinction between worldbuilding and storytelling is paper thin from an explicitly functional point of view. It's all about intent. Essentially, Worldbuilding is a form of storytelling---it's in-world documentation, in-world mythologies, background material, language information, all the rest. If you sit down and write a history of some fantasy world, you have both written a story (the Mythology, which is often an actual story or history in-world) and done Worldbuilding.
In modern fiction, the term Worldbuilding in practice is the part of the story that doesn't end up verbatim in the published novel, but informs it heavily and maintains it's consistency in the background, usually because it doesn't make an engaging story in its own right. In this sense, Tolkein wasn't Worldbuilding because he intended his history to be a worthy story in its own right. On the other hand, in practice, his history ended up being Worldbuilding to The Lord of the Rings, which was much the better for having the enormous history behind it.
Thus we can have a self-consistent answer. Did Tolkein think he was Worldbuilding? No, he thought he was writing a comprehensive history of a fictional world which he really liked, for it's own sake. In practice, was he Worldbuilding? Yes; Worldbuilding and Storytelling are closely related disciplines and look similar from the outside; their main difference in modern practice really is whether the resulting work is publishable or not. Did it improve the works that actually sold? Emphatically yes; the sense of history behind The Lord of the Rings is one of the things that really impressed me about it. Do authors sometimes think they need to imitate Tolkein and his (inadvertent) Worldbuilding, and thus catch Worldbuilder's disease? Yes. Did Tolkein have Worldbuilders disease? I'd have to say so. The fact that he was doing it for his own amusement or thought it might be publishable have no bearing on the fact that he kept on trying to refine the Story that represented his worldbuilding.