Actually at the CS department at my university we used RateMyProfessor to avoid truly bad professors, it was not just about grades you seem to be implying. We used it as a tool to protect ourselves as consumers from a business which was taking advantage of its customers by hiring trash and passing them off as professors. That's not to say all the professors were like that, but the CS department was being run that way. I'm not kidding - the school was even investigated by the FBI for the number of registered sex offenders the CS department hired as instructors.

So it was more like, "OMG! dont take professor smiths class. he harasses women and the school does nothing about it."