You can't evaluate how good of a writer someone is unless you already know what they are trying to explain. The writing has to be about a topic for which the evaluator and the writer have a common level of understanding. This means that post-modern literary analysis is useless for teaching writing. Everyone can interpret the text any way they want, and each individual is expected to interpret it slightly differently. The correctness of the content is subjective. If the correctness of the content is objective, and mutually understood, then any confusion added during the writing is bad writing. Driving the excess confusion and length to zero is basically the problem of learning to write well.
In practice, writing isn't used to convey mutually understood ideas, it's used to convey novel ideas. But when evaluating writing that conveys a novel idea, the quality of the idea is mixed in with the quality of the writing. That's good for teaching how to think, but it just adds another variable in the way of teaching how to write.