At "War of the 'iptivists" on Language Log, MJ asked in a comment, "What would a real prescriptivist look like?"
In context, he meant an actual, respectable, principled prescriptivist rather than a charlatan,* a fit counterpart to a descriptivist.
I essayed a short reply: "A real prescriptivist would look like an editor trying to make choices appropriate to writer, occasion, publication, and audience; informed about descriptivists' empirical findings, and respectful of them; and attempting to arrive at sensible judgments about which conventions merit following. Modesty forbids …"
Mark Liberman generously follow up with this: "[The] genuine scholars of English usage find themselves forced to spend as much time marshaling evidence against the cranks who promote non-existent 'rules' as they do correcting the barbarians whose prose is genuinely non-standard, confusing, or mistaken. As a result, the word 'prescriptivist' is generally taken to refer to the crazies rather than to the scholars, and this seems unfair to me. The scholars also prescribe, after all, it's just that their recommendations are based on a rational analysis of the facts. It's as if we called witch-doctors 'prescriptivists' because they prescribe on the basis of magical thinking about imaginary spirits, while calling practitioners of evidence-based medicine 'descriptivists' because their recommendations are based on the factual relationship between remedies and their consequences."
The key word is prescribe. Descriptivists produce grammars and studies and dictionaries with their empirical findings about how language is operating. They disagree with one another, as scholars do, but they are bound to support their assertions with their findings. I am happy to heed their reports, but as an editor my function is to say what the writer ought to do.
That makes me something of a rhetorician, responsible for gauging what vocabulary, what register, what metaphors, what syntax will best fit the author's subject and purpose and will be best received by the audience. Or, to come at it from another angle, I advise writers on questions of taste and fashion. Separating subject and verb with a comma was all the fashion in the eighteenth century; today it looks gauche. Contact as a verb was vulgar in the 1950s; today no one gives it a thought. I am the you're-not-going-out-in-public-in-that-are-you? colleague.
Tastes will vary, and judgments with them. Authorities, genuine authorities, will disagree, and students of the language will examine the authorities carefully, case by case and over time, to arrive at their own sensible judgments. Writers who have a choice will seek out editors whose tastes are congenial and whose judgments can be trusted.
So yes, there are real prescriptivists. I have worked with scores of them. I have trained some of them. They thing they all have in common, the quality that sets them apart from prating bogus prescriptivists, is respect for the evidence.
*Think I should give Clark Elder Morrow a rest? Hard to abandon such a delicious target, one who does not merely sport a "Kick Me" sign but has it embroidered into his coat.