juju plugins must support --description and --help, so that juju "help plugins" and "juju help $PLUGIN" can present them.
juju-backup does not:
$ PATH=./usr/lib/juju-1.21-alpha3/bin:$PATH juju help plugins
...
ERROR 'juju-backup --description': exit status 255
Juju Plugins
Plugins are implemented as stand-alone executable files somewhere in the user's PATH.
The executable command must be of the format juju-<plugin name>.
backup error occurred running 'juju-backup --description'
...
$ PATH=./usr/lib/juju-1.21-alpha3/bin:$PATH juju help backup
Connecting to machine 0
Permission denied (publickey,password).
ERROR subprocess encountered error code 255
ERROR subprocess encountered error code 255
Bug #1391276 shows that juju-backup is dangerous, therefore running it when you only want to list plugins or get help is a serious issue.