What the directions don't state is that setting this to No enables anonymous visitors to read not only teacher profiles but also the profiles of any students enrolled in courses that have guest access. This may be a privacy issue.

The effect of enabling Force users to login for profiles is that anonymous visitors cannot read the profiles for the teachers in a course that accepts guest access. They must register as a student before being able to read student and user profiles. This may be a drawback if your teacher profiles are a selling point for the course.

Consider enabling this to force people to register before reading student or teacher profiles. Then, if your teacher profiles are a selling point, you can add a section to the front page for About Our Teachers.