Hello,
The discussion so far has been about biographical data on Wikipedia and licensing data on Commons, but other projects have their own needs for it.
Wikisource, especially, is in desperate need of metadata. We have some 140,000 pages on the English wiki alone that represent poems, chapters, tables of contents, and so forth. These are essentially disorganized: we have human-usable templates and categories, but there's really no good way to find works besides searching their titles.
A few years ago we combined our metadata templates into two standard templates, {{header}} (for works) and {{author}} (for authors). Every single page already provides metadata to these templates, so implementing a metadata format for machine use is trivial once it is available on MediaWiki. We *really* want this; it would allow us to index our jumbled pile of works and authors in all sorts of very useful and interesting ways. Just a few example are author search and autocompletion (we currently list works manually), finding works by genre and year and subject and so forth, searching work descriptions, and distinguishing works from subpages.
Both formats have their own advantages and disadvantages. Microdata's simplicity is a significant advantage, but RDFa's built-in validation is also nice. Whichever format we choose, we'll make it all work behind the scenes in the murky depths of our templates. But it would be nice if you'd include creative works, authors, navigation, and indexes in the equation. There's more here than biographies and image licenses. :)