(replying to self) Ok, I see now one area where it actually seems to make much sense and looks like it can work very nicely: for stuff like git, vim/emacs, bash, etc. Generalizing, I'd say: for apps/services which happen to have abysmal, irregular UX/API, which often accrued/emerged instead of being designed. A classical reference manual just doesn't work good enough for them, because the "natural" edges/borders in the API are too random and accidental. "SO Docs" as it is now can then organize the chaos, while making it easier to categorize & improve than a typical wiki.
http://stackoverflow.com/documentation/git/topics
http://stackoverflow.com/documentation/vim/topics