I can understand that perspective about being used as a metric to the dean, but as a student I would have loved access to that info. It's kind like any other review system, a couple of bad reviews among many high reviews, you take that into account. but a lot of bad reviews, I'm probably going to avoid that professor, restaurant, product, service etc.

It should not be used as precise metric of performance, but as a general trend overall. Instead of one friend giving a bad review for whatever reason, now I have access to the hundreds of bad experiences or the 99/100 good experiences.

As a student I really don't care about the dean professor relationship. I am a paying customer of the institution. I want to know if that professor is worth taking their class.

Also if you don't care about students giving praise/negative reviews to a friend, that kind of says something.