The danger of Mentors leaving a Team (or losing their Mentor role) is that some Skills on Learner's Cards may be left without Mentors who can sign them off. While good authoring practice recommends that every Skill have at least two Mentors, sometimes all of them leave.
If any Skill is left "unsignable" because it has no Mentors, the Skill will become shaded pink. Team Admins will also be notified (configure this notification in your Profile > My Notifications and My Reports).
Learners also see the pink shading in Open Cards available for new enrollments. While Authors and Admins should always try to replace missing Mentors as fast as possible, Learners will still be able to enroll in new Cards and work on the other Skills that aren't pink until missing Mentors are replaced.