I am taking one AC outside of my specialty this semester and it hasn't been an enjoyable experience. You could argue that part of that is that I am not really interested in the area the class belongs to and that would be true to a certain extent. I am not interested enough to do research in that area but I still find it interesting in a general sense. There are other areas, mmm say, String Theory, that I could go on with my life without ever taking a class on it and be 100% happy. The problem comes, I think, from the fact that the class is taught by an expert in the field, and as such, he tailors the class as an introduction to the students that will end up doing research in that area. In other words, the class got too technical and too specific too soon. We could be learning about the general theory of that field, but instead, we are going over specific problems that are important to those who will work in the field the professor works on. It is similar to a professor who works on graphene teaching condensed matter and mostly focusing on stuff that will make a future condensed matter student able to work on graphene. The stuff might be useful, but the approach is too specific and for those who are taking the class to fulfill the out of specialty requirements it isn't fun. Nevermind the fact that in many cases in order to work on those current research problems you need previous knowledge that you were supposed to get from that class, but that the professor (apparently) assumed everyone already had.
I do think throwing in some current open questions is important, it gives students a better idea of where the field is going and why. But there needs to be a balance.