We had a problem with teachers being surprised seeing all these extra (system-assigned) people when going into participants.
Hiding them altogether I felt was not the best solution as we like transparency - teachers should know who can look into their course.
My fix which I've just come up with is very simple - change one default value near the top of the moodle/user/index.php file...
$roleid = optional_param('roleid', 0, PARAM_INT);
$roleid = optional_param('roleid', $CFG->defaultcourseroleid, PARAM_INT);
I think it is entirely justified to by default, show the default course role (instead of ALL roles). Now, teachers will have to specifically select ALL to see ALL. No scary surprises and no secret assignments.