I agree with @jb510

There is no need to be dogmatic about how one should use GitHub.

However, if you want to share your plugins and themes throught GitHub as an alternative to wordpress.org, you are certainly right.

If you do have one repository per plugin/theme you can install and update them with
https://github.com/afragen/github-updater
https://github.com/franceimage/wp-alt-repositories
and yours obviously :)

Thanks for the .gitattributes reminder by the way.