One of the greatest benefits of wikis is the liberal interconnectedness of the pages. Inflicting categories implies a somewhat hierarchical structure, whereas interlinking allows freer connections between pages. There are more ways to relate pages than just by subject matter; ideas can reference each other across any particular boundary somebody might set up.
The WikiEssence is to let things grow organically rather than attempt to impose order and structure. The idea behind refactoring and DoTheSimplestThingThatCouldPossiblyWork is that it is much easier to manage complexity afterward rather than try to predict it beforehand. Thus, let people spill their thoughts wherever they please. Interlinking will make those thoughts accessible where they need to be. Later, categories might help you find a page, but then again they might just waste the categorizer's time. One should wait until a page has settled down and solidified before sticking it here or there.
Ultimately, Wiki is not a forum, and I think it's a shortsighted idea to shoehorn it into that mold. Wikis can do so, so much more. Hang out here and absorb the Wiki spirit. It's the cure for all of one's compulsive tendencies.
-- JohnKugelman
[Discussion of "categories" and "somewhat hierarchical structure" moved to LimitsOfHierarchies.]