It would be nice if Moodle provided triggers that admins could set in any context, which would cause a specified role to be automatically assigned when a user with certain characteristics entered the context. For example, one of the characteristic could be authentication method. This role could be in addition to a default role (e.g., Auth user, on entry to System, Student on entry to course), or in addition to a default role.

See the following discussion for a use case
http://moodle.org/mod/forum/discuss.php?d=92537