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?