This is proper. Those who have knowledge should encourage others to seek it by sharing their opinions. However, they cannot share their knowledge itself, for it is derived from their personal experience. One can relate ones experience, but cannot truly share it. For this reason, we are bound, as teachers, to share opinion and testimony.
Testimony is the declaration of deep experience and conviction, which great teachers possess.
Teachers hope that in the sharing of opinion and testimony they can motivate and inform the student to the point where the student moves beyond opinion, to experience, and then to knowledge. It is a process of transformation. It is the difference between an educated person and an opinionated one. We now come to the second half of the quote from Idries Shah. If students knew what they needed to know and how to learn it there would be no need for a master, a guide, a teacher.