I was chatting with haut this morning, and this thread came up, and she made a comparison I thought was really smart.
There's an attempt to reify grammar into a certain set of rules, but many of those rules date only to - eh - the early 20th century, when people started writing grammar text books. The pet peeves of a few individuals (dislike for passive voice, split infiinitives, and so on) have beeen interpreted as dogma when there's plenty of historical precedent for the correct, English use of things like split infinitives. (English is not latin; it is not even latinate; it's a bit silly to assume that it should be regulated by the same rules that regulate latin.)
Anyway, haut's comparison -- grammar isn't a set of absolute rules, regulated like some sort of mathematics. It's more like the rules that govern the composition of music. You play it by ear, to make the words flow in the right rhythms. You do so within a framework, of course, just as music exists within a framework, but the right music will know not only when to follow the rules but also when to break them.