FFS. It's not more "lightweight" and it's not for storing that type of information. The only plugin that I can see of all the plugins that has a logical reason to use MySQL would be LogBlock.
Well, for one thing it makes it easier to manage stuff if you're also using the site as a manage tool for admins that aren't online, or for people to register/get vouched (to get building rights), and stuff like that. edit: Believe me we've used a yml flatfile reader/writer up until Permissions 3 came along. It used to lock up our server quite regularly when reloading the worlds yml files. (now we just use jsonapi in combination with the MySQL DB to manage permissions. Makes our life (and the server) a lot easier)