It would be useful to edit protected pages for interface admins (including main page and templates). At least some of them are what I would call part of the interface.
Most of the pages like that are technically complicated and some use HTML. Many times they require technical knowledge to edit (HTML, CSS etc). So interface admin seem like a natural person to edit such pages. This would be especially important when doing larger refactor (when you e.g. need to coordinate changing structure and CSS or structure and JS).
Also note that having interface-admin permission I can effectively change every page. Having rights to global JS allows doing almost anything with content. Obviously that would probably not be seen as a good contribution 😉, but still.