They are done by the people who have been involved the most in the work. Our experience has confirmed that this method results in a fair assessment of each student's efforts. A student is usually well aware of his or her overall capabilities. We tell students to be generous as well as realistic. And they are.
Students know that they are setting themselves up for a fall if their evaluations are unrealistic.
We are flexible in reporting, too. If parents and students have designed a better method to report to us, they are free to use their self-designed method as long as it provides us with the information we need to keep the student's transcript up to date. The real proof, as the saying goes, is in the pudding. We all learn at our own rate, whether or not we have someone looking over our shoulders and prodding us to do just one more math problem. Or whether or not we can distill what we have learned into a concise report on the subject.