There are times that I review without a comment. Sometimes a person just don't know what to say about it at the time, so it's just easier to give it a star rating without a comment. I also have my theories of why some may always rate without a comment.

Given the low end of the literacy scale evidenced by some reviewers, I`m just as happy this isn`t a requirement

You cannot post a review without rating it. Then it should work the other way also, you should not be able to rate anything without posting a review. Furthermore, rating should only be registered and valid if it is accompanied by a comment of the evaluation.