This whole series of posts just underscores why I don't like the words "descriptivism" and "prescriptivism." When one says one's a descriptivist, this immediately makes people think one doesn't want to prescribe. This is of course completely false, and I would have thought that my posts (and Eugene's) would have put that idea to rest. But no, this misconception dies hard.
Am I a descriptivist? Yes! Because I think usage is the ultimate guide to what English means. I'd think that even self-described "prescriptivists" would say the same thing if, as anthropologists, they encountered a new tribe in the Amazon and tried to describe their language. To know what the language means, you have to observe its practitioners and see what rules they themselves follow in speech.
Am I a prescriptivist? Yes! I've been an editor of a journal in the past (and so has Eugene), and I still act as editor when I read friends' drafts and my students' work. When I write an article, I send it to Eugene, who tells me how I should rewrite it. Heck, Eugene has even written a book called Academic Legal Writing, in which he gives the reader expressions to avoid!
And it's clear why we're interested in prescribing usage: In my case, my only rule is to speak in ways that make you best able to accomplish your goals. Since my goals are usually communicative, I believe in speaking in ways that are clear and comprehensible to my target audience. (And since my target audience often changes, the content of "clear and comprehensible" also changes.) Anyone's "rules" are only valuable to me insofar as they serve my goal. But once I've stated a goal, for instance effective communication with and persuasion of legal academics, there is probably an objectively best way to pursue that goal.
Therefore, to the extent a particular phrase makes my thought unclear, marks me as uneducated and therefore reduces my credibility with my readers, or something else along those lines, then using that phrase is a mistake — because it's a less effective way of pursuing my goal. (When people correct language mistakes in my posts, most of the time I myself would agree that it's a mistake!) The best way to pursue my goal might even be formalizable by means of rules — and most of these rules are indeed the ones we learned from our 7th-grade English teachers — but there's no necessary relation between the one and the other, and of course, in case of conflict, it's the English teacher's rules that should go out the window.
So the notion that I don't think there are better and worse ways of speaking — that I wouldn't teach my kids how to talk and how not to talk — is silly. The difference between self-described "prescriptivists" and "descriptivists" isn't that the first gang prescribes while the second gang describes. When I say that my students are speaking or writing incorrectly, I mean that they're expressing themselves in ways that I don't think are likely to achieve what I think their goal might be (and of course I have to explain why the words they use are ineffective). And when I choose how to speak, I likewise choose the words that I think are most likely to achieve my goal.
This "functional prescriptivism" business is a difficult exercise, and miles away from the "anything goes" that some people use as their caricature of descriptivism.