Rob,
It seems to me like the permissions you are allocating to the 'Share Rubrics' role have the side effect of forcing access to assignments. This is perhaps not entirely surprising since Rubric is a part of the Assignment activity and therefore to access a rubric you're accessing an assignment. Normally this would not be an issue for an individual course with a Teacher, but when you make the role with it's capabilities system wide it would mean that all teachers would appear in all gradebooks which may indeed be the case. Are there cases in your Moodle site where a Teacher with the Share Rubrics role does not appear in a gradebook?