What's "correct" English?

A reader wrote:

I'm sorry to disagree with some of the things you tell people, but one of your answers on your homepage is too obvious to be just ignored. You state that  "In informal English, most people would say "Neither Joan OR Jane LIKE sushi." That's all right in conversation." This is certainly bloody nonsense. I don't think you should advise people to say  "neither.......or", and it's certainly NOT all right in conversation, but just very bad and uneducated English  ........just as using a plural verb with a singular noun makes no sense. It's definitely not "all right in conversation", and it's certainly not just in "formal writing" that one should use a singular verb with "neither".   -perhaps in certain parts of America, but never in England.  I think that what you offer people here is very poor advice,telling them to be content with nonsensical phrases. ......or perhaps you think I haven't got no sense of no conversational English nowhere ??   And I replied:

In conversation, it takes seriously crippled social skills to keep correcting other people's usage--especially if they speak a different dialect. I can do it (one of my incarnations is the English Inquisition, which no one expects), but what's the point? I wasn't advising people to use "neither/or"--I said that most people, using informal English, would say it. And in talking with such persons, I'd say "neither/nor" and they'd understand me just as I would understand them. Unless the topic of conversation were the superiority of my particular dialect, I would no more correct their usage than I would put salt and pepper on their eggs. Whatever the usage might be in England (and which parts of England, and which classes?), English has long since escaped into the larger world. English in Singapore or South Africa has its own usage rules, mostly unwritten. A young woman in Iran sends me her poems in English; I correct her usage where she's ungrammatical or simply confusing, but Farsi-flavoured English is just another contribution to the language. Even the Oxford English Dictionary is descriptive, not prescriptive. It's the biography of the language as it was written, spoken, used and abused over a thousand years, not an outline of what English would have been if only it had been properly managed. So while I don't agree with your basic position, I'm grateful that you took the trouble to write. I haven't posted anything new on Ask the English Teacher in ages, and you've raised a point worth discussing with a larger audience.