This is a really important point, and it'll probably justify its own blog post soon, but a quick response would be this:
I'd class United 93 and Black Swan as being, sort of, emotionally exhausting. And yes, I'm not going to rush out see them again. But that's very different to saying they're a bad film. I would suggest that, at high levels of quality, low levels of rewatchability makes it actually a better film, in a way.
Or, at least, it differentiates them from another terrific film like The King's Speech. If you're the kind of person who likes watching beautifully constructed but largely enjoyable films, your tastes will tend toward the top-right of the graph. But, if you enjoy more difficult or harrowing films, your favourite films might be lower on the rewatchability score, on average.
My point is, while quality is quality, a measure of skill and talent, rewatchability is personal, and groups similar films together in interesting ways. Where your favourite films come out on the graph is really not important to anyone but you, when you're looking for more stuff you like.
You may still disagree, but thanks for taking the time to comment :)
-glen.