Pages like https://stats.wikimedia.org/EN/TablesWikipediaPT.htm have a lot of useful data, and as far as i know, there is no other place where they are easily accessible. Providing an JSON output would allow the reuse of that data without HTML scraping.
Closing this ticket as Wikistats version 1 is dead per https://stats.wikimedia.org/Wikistats_1_announcements.htm . In case this ticket is still a valid bug report or feature request for Wikistats 2, then please reopen. Thanks a lot!