Can people ever really be taught to write well? Or is it just something we have or don't, like being double-jointed?
I don't think that writing is about writing per se, but about how one is taught to think and express oneself. This, for me, was the great value of my hugely expensive (and paid for with student loans and scholarships) liberal arts degree and more professional Masters degree: it taught me how to think my way through things and express them well.
(Side note: the art of thinking through a problem is something I believe gets lost in our new testing-at-all-costs educational culture.)
There are also different types of writing: I consider myself a horrible poet, a decent persuasive writer, a hack reporter... So it depends a lot on the kind of writing a person wants to do, in my opinion.
That said, some people don't think clearly, even to themselves, and writing and communicating requires, for many people, linear thinking to be understood. "The Sound and the Fury" would be virtually impenetrable if Benjy narrated the entire thing, for instance. Some people aren't good at making a point, explaining a thought, making you see from their point of view, because they can't take the mish-mosh of thoughts that go on in all of our heads and translate it for a more general audience. I think that is teach-able, but maybe not to everyone.