Every developer needs to be on GitHub, whether or not they contribute to open source, and whether or not they have repositories (repos) hosted elsewhere. Seriously.
In this day and age no developer should have to justify using open source projects. Further, as a developer you absolutely need to read the code for any open source projects that you use; track issues and changes to the projects; and post issues when you encounter them so that they can be fixed. If you can contribute code fixes, improved documentation, and/or code enhancements back to the community, even better.
Source: New feed