Everything is of limited use.
Well, obviously. The implication was 'of sufficiently limited use to be notable', or 'of relatively limited use'.
Rules cited by opponents of proper grammar instruction as ridiculous are "don't split an infinitive" and "don't end a sentence with a preposition".
No, these are rules cited as ridiculous by opponents of bullshit grammar instruction. Proper grammar instruction - of the kind you advocate below - wouldn't teach them.
But these rules have not been taught for thirty or forty years.
This simply isn't true. I've been taught them for years, and I'm still being taught them now by multiple members of my English faculty. They're so thoroughly still being taught that I sometimes even follow them because I know the supervisor I'm writing for prefers it ugly but 'right'.
The average American high school graduate these days doesn't even know what infinitives or prepositions are, or even to what branch of knowledge they belong. That is the kind of grammar instruction I am in favor of.
Me too! One of its many advantages is that technical knowledge allows you to think more concretely about the many alternatives open to you and judge which is the best. This is roughly equivalent to thinking 'should I apply that rule about the passive voice that I know isn't really a rule in this case?' only with a layer of silliness stripped out and a greater likelihood that you actually know what the passive voice is.
As for style rules, I agree that there are few that are universally applicable. But when the writer knows his purpose and function, style guidelines can help him deliver his message with more power. Prime sources for such rules are the Economist style guide, Orwell's "Politics and the English Language", and Gowers's immortal Plain Words.
Well, yes, when a writer already knows what they're doing any number of things can help them deliver their message with more power. They won't improve by following a style guide, they'll improve by reading it critically. Beaver's criticisms of Orwell are for the most part perfectly valid: if I take the first five rules of 'Politics and the English Language' at face value they will ruin my writing. That is the way it presents itself, and that is the way rules like these are almost always presented. This is exactly what his sixth rule is about: making it clear that the rest aren't rules at all.
But of course the best way for the ungifted to learn good English style is the same as it is in any art: the imitation of masters. (E.g. "Describe a tree in the style of Coleridge.") This of course is desperately unfashionable. Man.
I think that 'of course' is a bit presumptuous without any evidence, but that aside it's worth pointing out that a great many style guides (not all, by a long stretch) are roughly the equivalent of advising 'write like Coleridge, always.'