Apologia of a Reformed Grammar Nazi

This semester I took an introduction to linguistics called "The Unity and Diversity of Human Language." It was all interesting and completely new to me, but I didn't find any of the concepts particularly shocking until we started discussing sociolinguistics. I was suddenly informed that, linguistically speaking, there is no such thing as standard English - it is a socioeconomic construct of the elite, and it is therefore improper to place value judgments on different dialects of English because they vary in systematic ways and are all equally linguistically valid. I was profoundly disturbed. I've been a self-proclaimed "grammar nazi" as long as I can remember (thanks in great part to my mother). Suddenly, I discovered that the kind of grammar I'm so particular about has no linguistic basis at all - it's what linguists call "prescriptive grammar" (as opposed to universal grammar), and in terms of the biological basis of human language, there is no reason for it to exist. Rules of prescriptive grammar are the kind you learn in high school English - don't end a sentence with a preposition, don't split the infinitive, use "whom" in the objective case, etc. It turns out that the reason we learn these rules in school is because they're completely unnecessary to our effective communication in English (and by effective communication I mean day-to-day conversations with other speakers). Everything we truly need to know about English comes from the principles and parameters of universal grammar that facilitate our childhood language acquisition - and as far as linguists can tell, there are no parameters involving the usage of whom or split infinitives. Prescriptive grammar is entirely a sociolinguistic phenomenon, a set of language "fads" perpetuated by the dialect of the class in power. I'm not likely to forget all the rules of prescriptive grammar I've so carefully memorized, but you can rest assured that in future I will stop and think before I "correct" your grammar.