Lighter says: The point of teaching "correct usage" - when the sense of an utterance isn't at issue - is to keep the student from being thought illiterate or incompetent by future, unknown readers or interlocutors.
It may be the sole point.

Well said, Lighter, in most cases. However, I had to interview people in investigations, and then write up affidavits for them to sign. I that sort of situation, and actually in a large number of situations, correct usage can be extremely helpful. It allows one to be able to write succinctly and incontrovertibly - what you write can be understood in only one way, the way the writer intends. It allows one, for example, to write a legal document without all the "legal gobbledy-gook," because the writer has written incontrovertibly without the need for all that extraneous legal bullshit. And in 30 years, nobody ever questioned or corrected an affidavit I wrote.

That being said, maybe I'm a nicer person than "meself." I would have left the student's writing untouched, despite the fact that I would have considered meself's "each course HAVE" to be proper usage.

-Joe-