I was going to say that the problem from the instructor's point of view is deciding whether the student really has the necessary background, but Desrtopa is probably right that some sort of testing system could be set up.

In one sense, I agree that there shouldn't be any gating. It is overly-paternalistic. Students should be allowed to risk taking advanced classes as long as they don't gripe about their failures later. But on the other hand, the actual result that I see in my classes is that many -- and here I mean maybe as many as half -- of the students in upper-division courses are not prepared to do philosophy at that level. They don't know how to engage in discussion appropriately or productively; they don't know how to write clearly or criticize arguments effectively; etc. If they only affected themselves, I could put up with it. But they don't affect only themselves, they affect the other students as well.