Besides which, I think exposing rating systems for courses creates perverse incentives. It's been well studied that students who get better grades rate courses higher. Is that really what we want to be measuring professors against?
OK, but now you're arguing that Yale students shouldn't be exposed to free expression because they're incapable of handling its consequences. I think that's silly, but fine: it's a real argument. But let's stop pretending this kind of paternalism is consistent with even a modest notion of free expression, because it's very clearly not. It says "you can only say things we approve of, because we know best." Being right about knowing best doesn't make it not a limitation on free expression. Free expression as a principle necessarily doesn't hinge on whether you think the individual expressions are right. That's, like, the whole point of it. [1]
You're also arguing that 1-5 rating systems are silly and Yale shouldn't bother collecting them, but that has nothing to do with the YBB+ website or how students will interpret it. For example, wouldn't those perverse incentives you mention be in place already, given that providing feedback to professors was the whole point? Ratings don't magically transform from useful evaluation tools to misguided oversimplifications just because students have convenient access to them. Either they carry useful information or they don't.
[1] I'm not taking a free expression absolutist position here. (I'm tempted to, but it's totally unnecessary for this discussion.) The point is that to the degree to which you make rules about what kind of thing people can express is, well, trivially the degree to which you limit free expression. And limiting the ability of students to transmit truthful data about the courses they're taking is a very strong limitation, and it would be hard to formulate a consistent, principled policy that permits this kind of restriction and doesn't just say, "you can't say things we don't like".
I don't really have a bone in this fight, so to speak, but I do find it interesting that the Dean of Yale has such a problem with the way their students are using their data to express an idea, when their very own Freedom of Expression Policy says:
Yale's commitment to freedom of expression means that when you agree to matriculate, you join a community where "the provocative, the disturbing, and the unorthodox" must be tolerated. When you encounter people who think differently than you do, you will be expected to honor their free expression, even when what they have to say seems wrong or offensive to you.
Although, if we're being pedantic, their policy technically states that they expect their students and graduates to honor others' free expression, not that they agree to honor their students' free expression.
Odd, I read jsun's comment to be claiming that the professors were incapable of handling the consequences, not the students.