Good question. And I note someone else is asking the same thing.
I have also been looking for some clarity on this so I could add some structure to the wiki-related content on the MoodleDocs wiki, which I think should also perhaps cover wiki theory/philosophy in general in case people are working with 3rd party wiki's (e.g. Wikipedia) or wiki tools outwith Moodle, as well as reducing duplication in documenting these areas when writing more specific documentation for both dfwiki, and the standard erfurt based wiki.
I can at least offer a part answer on MediaWiki, which is the software used to power both Wikipedia and the MoodleDocs projects. I would guess that it is not suitable for use within Moodle as a resource because it was never designed for that purpose, and a fair bit of wrangling would be needed to make it fit in with the rest of the Moodle code.
There's been a couple of questions about Mediawiki/Moodle integration in this forum but I think they are a bit of a red herring in this context. What has been done is simply to link the user authentication systems so that a) automated spammers will find it more difficult to deface the MoodleDocs wiki, since it is no longer a standard installation and b) Moodle.org users will not need a new username and password to contribute to the wiki. This is only appropriate if you are envisaging one large wiki for your entire Moodle site, which sits outside/alongside the Moodle installation. The other two wiki options are structured as Moodle activities, and so you can have multiple independant wikis within each Moodle course.
