We have class tested this material and found that the examples become unrealistic or poorly behaved if we omit any of these topics. Moreover, in class testing we found that students respond to early classes in basically the same way that they respond to early functions. Certainly the material presents some problems. However, they are as capable of learning it early in the course as they will be later in the course. Moreover, covering classes early leaves students with a better working knowledge of classes.
Students have a strong loyalty to the first technique they learn for solving a problem.
So, if you want them to really use a technique, then you need to teach it early. Moreover, classes are not the hardest topic the students encounter. This is discussed in the section of this preface on flexibility. This book was written to allow instructors wide latitude in reordering the material. We would urge instructors to move along this path quickly, but that decision is up to each instructor.