What detractors of prescriptivism object to is the attempt by individuals to impose artificial and arbitrary rules on usage.... The history of English shows that language changes under the influence of good writers and speakers, not of academies of the French model.
For those of you uninitiated into the mystical, cauldron-brewing holy war betwixt honorable prescriptivists and diabolical descriptivists (I take no sides of course), the OED defines prescriptivism thus:
[T]he belief that the grammar of a language should lay down rules to which usage must conform.
For further guidance I have consulted Scripture, 3rd ed.--Fowler's Modern English Usage--but alas this indispensable and generally uncompromising work observes only that the division between these two schools of thought is "one of the great linguistic battles of the 20c."
As my above, not-too plausible deniability suggests, I lean toward the prescriptivist school. I once put my shoulder into it, but that's a difficult stance to sustain, since, as Narayanaswami correctly adds, "Some of the rules are violated even by writers who champion them." I don't know where Shakespeare would have stood on this beastly battlefield of Ps against Ds, but writers come no greater than Will, and he was forever reinventing the English language.
Words, grammar, usage, conventions and heresies--all are a religion to some of us. And on this, as with the real thing, I declare myself profoundly agnostic. I'll wait and see how things turn out.