You can look over Prof. Bruce Macfarlane's book, intellectual leadership. He largely discussed the graduate supervisory roles of faculty members, within the dimensions of Mentor, Acquisitor and Role Model. See the refs in the paper attached.
I believe that the most important role for higher education is to equip our students to spend a lifetime engaging with the contemporary avalanche of information overload in a critical and informed matter. Accordingly, I believe it is essential for professors to go beyond the content of books to model the critical thinking we hope to develop in our students.
As long as university policies are followed and student expectations are clear; faculty, especially at the graduate level; should be able to make decisions based on student body. At the graduate level, learners are adult learners, seeking new knowledge; additional information would be welcomed