From Adrian Lang's email to wikitech-l http://www.gossamer-threads.com/lists/wiki/wikitech/452196:
- The shims for $wgExtensionMessagesFiles as generated by
maintenance/generateJsonI18n.php register a handler for the hook
LocalisationCacheRecache when included
That is true even in 1.23; LocalisationCache still includes the shims to get non-message data "For backwards compatibility [...] Variables other than $messages still need to be supported though.".
However, in practice, non-message data are contained in separate files; it is unnecessary to load the shims.