The terms of the GFDL require certain access to source text. Any element of the GFDL text corpus must remain available in source form to anyone, and no page in it can be distributed without the URL of a source form that can be instantly retrieved by anyone. This includes those "banned" by the sysop power structure. Mediawiki provides this with the Special:Export facility in recent versions, and for unblocked IP's with "edit page" or "view source" links on every page.
The GFDL guarantee also includes those who do not intend to use the mediawiki software. Bulk dumps are available only in MySQL dump format, which can be difficult software to run, and, is certainly not suitable for most purposes of a large public wiki (text searches, for instance, take an extraordinary amount of compute and disk time).