The problem is there's a good chance of measuring the wrong thing.

If you need to pass a test to post, sure you'll keep out a lot of bad comments. But you'll also limit the pool of acceptance. There could be plenty of people who fail the test having something positive to add. Standardized tests prove this. There are those who are lousy test takers but end up being very successful. Also, certain topics might be easy to answer with someone with sufficient experience in the subject without requiring even average intelligence.

So this limitation might increase the quality of posts, but also adds a kind of tunnel vision to the site.