(My theory, which I posted on Metafilter a minute ago. NB I briefly discussed this topic w/ Sarang some time ago. I lost the exchange but he has probably influenced my views. Note also that there is a pretty good Language Log post on this subject.)
There is no prescriptivism. There is no descriptivism. These words don’t describe mutually exclusive opposed positions. If you try to define them rigorously you will not be able to cleanly distinguish them. I think we owe their vernacular ubiquity to the success of Language Log (note that I’m not claiming LL invented them).
“Prescriptivism” in particular is a kind of culture-war tag for a certain family of popular, non-professional language criticism, like “Eats Shoots and Leaves” or “The Elements of Style.” These works are often pedantic, and they often include stylistic pronouncements based on usage errors or false generalizations. Furthermore they often include a more-or-less explicit social conservatism (language was better in the good old days; it is okay to criticize poor people for deviating from Standard English). So the fit isn’t one-to-one, but I think it makes sense to think of “prescriptivism” as a label for a family of populist linguistic-conservate rhetoric.
If you are a blogger and linguist, then, you have at least three independent reasons to come out against “prescriptivism”:
1) Its practitioners are wrong on usage points.
2) It imports a conservative agenda.
3) It usurps professional privilege, because its practitioners are not trained as linguists.
I don’t think I’m being unfair in suggesting that all three of these reasons from time to time may motivate attacks on “prescriptivism.” And we can generalize 3) so that it applies not only to professional linguists but to educated liberal people generally:
3a) Prescriptivism represents vulgarity in the public sphere, because the uneducated and unsophisticated are usurping the privilege of the educated.
(Note that I think a lot of liberal criticism of populist conservatism takes this form — “that’s sexist, as you would realize if you’d taken Women’s Studies 101,” “that argument relies on Econ 101 reasoning.” This is part of the psychological appeal of painting the opposing masses as racist, as well. Part of the reason “but I’m not racist” is an unpersuasive argument is because it signals unsophistication. Conversely, theories of what kinds of behavior are racist or sexist have become at least formally more sophisticated over the decades, as the Republicans became the party of whites and anti-racism became the distinguishing social feature of liberals.)
So much for “prescriptivism.” The thing that bothers me about the culture-war usage of these terms, though, is that the activity of linguistic prescription gets subsumed under the label of the supposed thought-system of prescriptivism.
Prescription is merely the activity of offering guides or rules to language and usage; it doesn’t depend on those guides and rules having any metaphysical status. You don’t have to be a “prescriptivist” to see the value of prescription. For example, languagehat, no prescriptivist, can say:
Which is not to say I don’t appreciate the pleasure to be had in manipulating the traditional rules; you will notice I write according to them, and I make my living from them as well (I’m a copyeditor).
The job of a copy editor is in part linguistic prescription. Nor, I think, do prescriptions have to have high statistical validity as descriptive linguistics to be useful. It is even possible, for example, to like “The Elements of Style,” as I do, without any particular metaphysical commitments. I don’t want to stake my whole argument on this, but it has always seemed like a useful and witty book to me. Geoffrey Pullum (of Language Log) obviously hates it, but he sees it as a symbol of the hated class, prescriptivism, and that makes him a very uncharitable reader of it. Get away from this essentially political division and there is still much to appreciate.
In sum, I think the whole “prescriptivism”-”descriptivism” opposition is an example of the ways that bloggy communication and political self-segregation limit and impoverish our discourse. It’s a contrived distinction that makes it easy to signal your loyalties but harder to appreciate the world as it is. That’s a loss for all of us.