What is interesting is that these assertions are invariably offered without a scrap of evidence by individuals with well-deserved reputations for analytical thinking. If someone offered such unsupported arguments in a research seminar, most of us would dismiss both the arguments and the arguer out of hand. In discussions of teaching, however, we routinely suspend the rules of logical inference without a second thought. It's not as if data on student ratings are lacking. So, for the record and in case you happen to find yourself on a committee where student ratings come up, here are some facts to throw into the conversation.
Students lack the wisdom and experience to evaluate the effectiveness of their current instructors.
Those who give instructors low ratings at the end of a course will in future years appreciate those instructors. If professors in your department who know how you teach rated your effectiveness, the results would probably not differ all that much from your student ratings. Student evaluations are just popularity contests. Even if student evaluations have some validity, there's no value in the multiple-choice forms used to collect most of them. You've got to interview students and ask open-ended questions for the results to mean anything.