Redmine

Indeed, only the 'main' role is displayed (based on roles order) to keep it simple and so that users can be grouped by role.
How do you expect this view to display all the roles?

I expected that a user is displayed under each role he is assigned on.

The purpose is that in our installation the roles are not hierarchically ordered. Manager is on top, but Developers and Supporters are nearly equal except for the workflow of issues. We follow the concept of verified issues. So Issues status is New -> Assigned -> Closed(resolved, integrated to SVN) -> Verified (which is closed). Only Supporters who are testing a software release are able to "verify" an issue.
In some cases Developers might be Supporters an vice versa.