+2
Well, I think many, or even all, agree that slapping a number or letter on a movie as a way of judging it, is far from optimal, but I do it mostly as a way for myself to remember how I feel about a film, so I don't have ro read several paragraphs to find out.
To judge only by a rating is definitely not the way to go, but it's a small and easy way of quickly categorizing something for yourself or others, so they have an idea, and if they want a clearer picture they can read the review.
And by the way, my math example was for fun, so you shouldn't take it as serious as it seems you're doing. And actually you can't "measure the result" in the example I was using, since I referred to an on oral math exam. But anyways, don't think you have to put everything up so nicely and all, because I would bet several people agree that simple gradings are far from perfect.