I would think that the most evident point would be that some students simply are not ready to make the choices to understand the scope and sequence of all that is needed to be successful.  Students have to gauge the entire focus of a curriculum.  Seeing that this is something in which they are learning, it might not be wise to enable or empower them to discard elements they find undesirable.  For example, in a subject like mathematics, where sequential learning is critical for the acquisition of new and more complex material, students cannot choose to discard one aspect and expect to be successful with the rigor that follows.  For example, if a student discards fractions operations, they will be unable to be successful in any rigorous element of mathematics instruction.  Another reason why students should not be empowered to make curriculuar based decisions is that it creates chaos in the instructional methodology of the teacher.  If one student seeks to eliminate fractions, and another one decimals, and a third one percents, there is no way comprehensive instruction can be given because every student could conceivably eliminate a part of the curriculum.  It is the function of the competent teacher to lay a path for all students to follow in the acquisition of content in this light.  A good teacher can leave choices on how students might wish to pursue certain aspects of the content, but the overall content must be grasped by all students in order to see them become stronger and more competitive in the field of learning.  No one is advocating that students not be given a choice to develop their own passion and zeal in content.  Yet, the idea that students can "choose" or "select" their own topics of a curriculum denies them the opportunity to understand all aspects of it so that an informed choice can be made.