Most of client programs shipped with MariaDB 5.2 / 5.3 do not accept the plugin-dir as an option, which makes authentication that requires a client plugin impossible if the plugin is not in the default folder.

Examples: mysqladmin, mysqldump, etc.

default-auth is absent too, but I'm not sure if it's required by any of currently existing plugins.

If you decide to change it, please also include mysqlslap into the list, it might be handy for testing sometimes.