1. If I go to the trouble of writing a textbook, I'd do it because I felt there was something lacking in the other available texts/materials.
2. There is really no way to force a student to do anything.
Disclosure: I list some printed texts and reference books as required on my syllabi. I do this because I give open-book, open-note exams; but I do not allow the use of electronic devices (except pocket calculators).
Very good point. In fact, I have noticed the exam same issue while teaching. We need to refer multiple books. There is no book which covers the entire course in an understandable manner. One has to then resort to writing his own notes for the class.
2. There is really no way to force a student to do anything.
Disclosure: I list some printed texts and reference books as required on my syllabi. I do this because I give open-book, open-note exams; but I do not allow the use of electronic devices (except pocket calculators).