It is a nice idea to be able to bulk backup/restore entire category to directory using cli.
I also thought it would be handy to name the backup file yourself instead of naming itself.
For that reason I have attempted to add below extra improvements to existing backup.php script:

1) --destination=STRING can be either a directory or file. If it is a file then the course is backed up to that specific file, otherwise the files are autonamed. If destination is not provided the moodle backup course area is used.
2) --categoryid=id if combined with --destination=directory then the script backs up all courses of category with id to the directory specified, autonaming the files.

Please feel free to use/comment/improve.

You should also read related discussion : https://moodle.org/mod/forum/discuss.php?d=394427