Textbooks; Why do you assign them?

An August 20, 2007 article from the Inside Higher Ed folks about whats spurs students to read their text books [Link to full article] raises the question in my mind as to why we assign textbooks for classes. According to the research cited in the article:

Four factors (not all of which professors can control) best predicted whether students would spend more time with the textbook: gender of the students, the quality of visuals and the quality of photographs in the books, and the extent to which professors link assigned textbook sections to lectures and other in-class work.

That last item is the real kicker for me. If the textbook is NOT used as a class learning tool, why is it required? Could there not be "suggested readings" instead of required texts? Better still, why not get students involved with finding and evaluating those suggested readings? As with all instructional methods, it would take a lot of time to implement and the instructor would have to be on her / his toes to make sure that students had the necessary skills to locate, evaluate and report on quality and relevant learning material for each course. But imagine what the possibilities for actual learning!