The problem is whether it was doing a good job. Sure, students may have felt they were making good decisions by looking at 1-5 average ranking, and yes, the UI was very pretty and made it look like it was providing you lots of information, but the fact is, a simple average of a single metric is not a good way to pick a course.

There are always hard, scary courses, and those classes are going to end up ranked lower because the material is harder or the professor isn't as "cool" or because a 20-year-old doesn't see the point in learning how to do proofs. Letting that become the dominant factor in course selection is not good for education (or intellectual freedom--what happens when a professor with unpopular views is ranked poorly and his/her classes become sparsely attended?)