JIRA has many possible uses besides software, and in different contexts you will eventually want to have different roles depending on the projects.

For example, you may have a project that involves external people, therefore you may want to have a "Customer" role and a "Vendor" role, however, there maybe more that may not make sense for all the other projects.

It would be nice to be able to select a subset of the available roles in the system, having a default set for all projects. An implementation could follow the "scheme" logic already implemented for workflows, issue types, screens, etc.