It would be excellent if administrators could login as any user role and profile combination to test out settings directly in the production environment.
This is similar to the functionality currently available where one can login as a user however
1. You wouldn't need to get the user to grant you access
2. You could test out role and profile combinations not yet assigned to a user
This would be extremely useful for verifying that users have visibility and functionality that they require BEFORE they complain that they don't have it.