Some of Casey's recent comments bring up relevant issues that I'm happy to address. So…
do I best serve my children (or my students) by not objecting to their use of the objective-as-subject-of-sentence form? Whether "Linguistics" likes it or not, there does seem to be a language of power...
Whether he serves his children/students best is not really a linguistic question. It's a parenting question if he's talking about his offspring. It's a pedagogical/teaching-philosophy question if he's talking about his pupils. The question of dative/accusative/nominative forms is linguistic. So a linguist can probably answer some of the basic questions he has about why his kids use one form here and another form there. Another linguist could give him an answer about whether there's any evidence that a parent objecting to a child's use really makes any difference. Not about whether it should make a difference, but whether it does. To that point: in short, correcting your children's grammar will have little effect on their speech. They're going to talk like their peers whether you like it or not.
You can probably tell them that they have misunderstood what a giraffe is, and you might stop them from saying 'giraffe' when looking at a zebra. And you'll probably be able to convince them that the past tense of eat is ate rather than eated. But only when they've heard enough other people backing up your story by example.
And by the time their language differs from yours due to dialectal differences they're pretty much done listening to your advice. This doesn't mean they're done learning. And it doesn't mean they're done shaping their habits of usage and their command of registers.
The question of serving asks if needs are being met, but to answer that requires some agreement about what needs there are. What are you trying to give them? Honestly, the use of I and me and us and we isn't going to make much of a difference in their lives. Graduate students and professors all around me are constantly saying things like spoke to my brother and I and For my colleagues and I to consider. Those forms in speech aren't blocking many people from succeeding even in paths that claim to be picky about language.
A few people decided to jump on Barack Obama for his use of I in accusative and dative positions. What's the lesson there for an English teacher? Obama has succeeded while using registers as he does. He certainly knows that the forms that are overlooked in speech are not as easily overlooked in writing. So he avoids them in writing and uses them in speech. And even when they're not overlooked in speech, the people who pick on them are tiny little toothless pit-bulls. They'll grab on and hold on but they are no more than a nuisance. And by nuisance I don't even mean something that we should try to get rid of or something that causes any real problem. I simply mean that the anger is impotent. Don't worry about it. Really. Shrug and move on.
Should students know the mechanics of different writing styles? Yes. Teach them to know their audience. Teach them to choose a tone carefully. Teach them to use strategies for clear communication. And in my opinion you should do yourself the favour of separating writing from language. For that matter you should be careful to separate all edited and prepared expression from language. Teaching strategies for writing and reciting is a rhetorical business. That's fine. Linguists have nothing against such instruction. Most linguists are pretty good at it and many are happy to give it.
To Casey's next comment:
Whether "Linguistics" likes it or not, there does seem to be a language of power...
Linguists know this. And study it. Descriptivism is about understanding the structure of language. We have faith that an honest and unbiased description will lead to the conclusion that the power is determined by social forces and not because of an inherent quality that one dialect has over another. And reaching that conclusion does not mean that a linguist will then be determined to influence that power. I believe that is the primary misconception regarding descriptivism.
Casey then argues:
grammarians are no more or less correct than teenagers who are always introducing new forms.
And I agree. That's true as far as the grammaticality of their sentences goes. The active voice, for instance, is no more or less grammatical than the passive voice. If I choose to pick on "grammarians" it's going to be because of their claims that they are more correct than those teenagers. Or because of the logic they use to argue in favor of a form. I have no problem with William Safire sharing his opinions. I criticize him when he makes an assertion that I know to be false. About the history of a word or about the current state of language or about the forces at work in language change. The irony is that if a grammarian was to flat out say 'I just prefer this form' or 'I just like it better because I had a crush on the teacher who taught it to me' or 'This form reminds me of the social circle of which I would like to be a part' or 'I like this pronunciation because I just like the sound of that diphthong' then I would have little criticism. Those are all pretty honest assessments of preferred forms as an arbitrary judgment.
It's when those grumblers accuse kids of having a novel disrespect for language and for endangering the future of 'good' English like no generation before them that linguists have reason to roll their eyes.
If we are sympathetic to the creative impulse it's because that's what all languages do and it's interesting to watch it happening. If we are tired of rigidity it's because a corpse can only tell you so much about the life it led. Autopsies are fine but they only entertain for so long.