If a student sees a celebrity they like on the cover of a textbook, I think they are more likely to open the book and peruse its contents. It is possible that by simply opening the book, some content will catch the students attention and they will become more interested in the subject. Academics also has the reputation of being "uncool", and placing a celebrity on the cover may lessen this stigma.
Celebrities have good intentions many times when they try to promote a cause. However, it is unlikely that very many students will be inspired to study a subject because a celebrity's picture appears on the cover of a textbook. Instead, celebrities should donate, as well as raise money for programs that will enhance students interests in learning about different subjects.
If I were taking a biology class and I saw an actor or athlete that I like on the cover of the textbook, it's possible that it would make me slightly more interested in the class at first. However, once we started actually getting into the material, I'm sure I would still not really enjoy the class.
A celebrity's picture on the cover of a textbook does not equal interest in a particular subject. Granted it would certainly peak a students' interest and get them to open the textbook. However if the subject matter does not have any appeal to the student then they will quickly lose interest in what is being discussed.