A prescriptive dictionary ennobles--it instructs on the correct or preferred usage. This is in contrast to descriptive dictionaries, which merely document usage--no matter how ugly, sloppy or illogical. So a prescriptivist would say "I could care less" absolutely is not interchangeable with "I couldn't care less". A descriptivist would say that, even though the latter means almost the opposite of the former, since it is used to mean the same thing, then it does mean the same thing. A prescriptivist would say this is disgusting linguistic moral relativism, and should not be countenanced. Prescriptivists are like Apple under Steve Jobs--they have taste. Descriptivists are like Microsoft under Steve Ballmer--all they care about is going with the volume of users.
Sadly, the vast majority of dictionaries today are descriptivist. This includes the famous OED which, in the popular imagination, is the final arbiter of meaning for the English language. For prescripvists, it may be the dictionary of record, but certainly no arbiter. I am a life-long prescriptivist.
Not that I am completely without appreciation for descriptivist dictionaries. In fact I think Urban Dictionary is one of the finest of its type. But for professionally curated dictionaries, I will always lean prescriptivist.
I was quite amused by this article, which explains how courts of law are starting to refer to Urban Dictionary. But when I came to this quote, I laughed out loud:
Jesse Sheidlower, editor at large for the Oxford English Dictionary, points out, however, that popular does not mean accurate. “People may like a word because it was posted by their friend or because it was funny,” he said.
Oh, the irony of it! An editor of the OED pointing out that descriptive does not mean accurate. While I acknowledge the hypothetical point that Urban Dictionary may be on the under-curated side, and some entries may be outright bogus, this still has a very strong flavor of a professional in a discipline that has allowed its standards to be undermined by modernism suddenly lurching traditionalist, when challenged from below. A bit like the deconstructionist English professor who bemoans the fading influence of serious literature in popular culture.