It makes a lot of sense to use GitHub if you want to publish experimental code. The modules that you'll never plan to publish as a full project.
When you manage a contributed module, everyone is free to participate. Besides, drupal.org has tight integration with git.drupal.org, so you really have to use it.
- Commit attribution
- Issue mentions in commit messages fed back to the mentioned issue.
- Vetting process for new developers.
- Project release integration (branches and tags).
- Module security reviews and project ownership transfers.
I do not think it would be better if you move your code over to GitHub and ask others to participate there. Besides, I'm really happy with the version control systems now we have and how they have advanced over time.
However, if you maintain any other Drupal addons, for example, an configuration file for an IDE, a site migration script, proof of concepts, or something like that, I think github would fit better.
Also, there is a requirement to publish your code under GPL if you want to use drupal.org git.
I also maintain some modules, and I add github remote handler in addition to the drupal.org one. When you push your code, you can simply push it to GitHub the same way.