Briefly, I think that both attitudes to the language have their uses.
I was struck by a comment of another poster recently: usage should be corpus-based but not corpus-led. (The corpus is the body of language that already exists in books, newspapers, on the net, etc.) We should look to how the language is actually used (descriptive grammar), as this is what the learner will meet and deal with in the real world; at the same time, a basic prescriptive grammar is necessary for the practical learning process (as you have stated) and to defend the learner against e.g. falling into inappropriate social registers.
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