The answer I think lies in this bug report in the tracker:
http://tracker.moodle.org/browse/MDL-11313
It seems that the switch role function is not tied only to the switch role capability, but also the assign role capability. Without both of them the ability to switch roles doesn't function.
Re: Grant ability to Switch Role without granting ability to Assign Role
by Brett Hinton -
Number of replies: 0