Right. Flesch-Kincaid doesn't measure quality but things like sentence length (and I've heard librarians argue that other measures are better).

It's just that--because it's baked into MS Word--Flesch the easiest tool for most people to use, especially if they're new to the concept of "readabilty" or the idea that you can even measure such things. I'd actually love to write about some of the other measures, but when I've looked into that in the past, I couldn't find a way to write about for a general audience.