I would say that it is unhelpful because its prescriptions presuppose the knowledge most of our students don’t have. What good is it to be told, “Do not join independent clauses with a comma,” if you don’t have the slightest idea of what a clause is (and isn’t), never mind an “independent” one? And even if a beginning student were provided with the definition of a clause, the definition itself would hang in mid-air like a random piece of knowledge. It would be like being given a definition of a drop-kick in the absence of any understanding of the game in which it could be deployed.
I agree. But rules in writing, more than any other, are meant to be broken. To work from a correct definition supposes that that definition is correct, easy to learn, worth learning, or not subject to change. Adaptability is key to good writing, and you learn adaptability best when you're comfortable, not being whipped by rulers. Tests of comprehension and knowledge are easy to pass for those who can memorize and conform.