I support it. In my experience, many people don't know about GFDL (at all, or enough), so that could really affect them.

I think this should be WONTFIX/INVALID. Default setting of MediaWiki is 'no license specified'. When a license is used, it is substituted for $1 in 'copyright': 'Content is available under $1.'. Without making separate messages for the licenses that are supported by the deault installer, this is not feasible. Aside from this, there are some (very undocumented) settings that 'do stuff':

(from DefaultSettings.php)
/** Override for copyright metadata.

• TODO: these options need documentation */

$wgRightsPage = NULL;
$wgRightsUrl = NULL;
$wgRightsText = NULL;
$wgRightsIcon = NULL;

/** Set this to some HTML to override the rights icon with an arbitrary logo */
$wgCopyrightIcon = NULL;

To product Wikimedia. Can't do anything here per comment 2.

Closing bug per comment 3.