Some of the defensiveness may come from professional pride... Teaching (and medicine?) is not as easy to rate as, say, a product on Amazon or an Ebay seller. Speaking as a sometime adjunct instructor at a research university, the "course evaluations" represent to me bringing corporate management theory to a theoretically more high-minded institution. As others have commented here, the pressure to publish lowers the priority of teaching for full-time faculty. Bringing in low-paid part-timers to teach is not a healthy response to this, and publicly rating/berating teachers is a band-aid on a situation already lacking in trust and respect.

In a healthy college of Yale's quality, course choices should come from competent individual advising of the students and student-to-student discussion, rather than from ratings and a scheduling algorithm.