I generally find rotation useful, but I understand why some romantic or competitive partners would choose not to, and that's fine. I also understand arguments in favor of looser rotation schemes that allow partners who rotate to nevertheless choose each other more often than strict rotation would allow. However, the most confusion and time wasting I've experienced in group classes has come from that sort of loose rotation, badly organized or executed. Whatever type of rotation a teacher wants in their class, having a definite plan for it and communicating that plan clearly to the students can have a huge impact on the smooth running of the class. It's one of the most important aspects of class management, IMHO -- or at least one that I've frequently observed causing problems when not handled well.