I have a degree in linguistics. The most important thing it taught me is that there is a widely believed fiction, almost like a religion, underlying prescriptivist grammar. For the sake of social advancement, if you have both the means and the talent, it’s generally necessary to learn a list of arbitrary but extremely complicated prestige markers for your language, to earn the approval of the self-appointed priestly caste of grammarians, in order to rub shoulders with the rich and powerful. An overly complex shibboleth.
It’s a mechanism to oppress the lower classes while maintaining the pretense of pure meritocracy, by declaring arbitrarily that the dialect which is already spoken and written in the homes of the upper class children is proper, and all other dialects are improper, then implying that the “failure” of lower class children to acquire the prestige markers is an intellectual shortcoming, rather than the absence of privilege.
Can you buy books and hire tutors to learn these prestige markers? Of course. Is there general agreement among members of this cult about what their own rules are? Sure. If you choose not to use them, is your English “improper”? Absolutely not. It’s different but equal, as long as your meaning is clear. I would wager that more than 90% of people do not go even one day without saying or writing some example of “improper” English, which is nevertheless understood perfectly well by the recipient. Successful transmission of the message is the only true test of linguistic legitimacy. Everything else is performative.
By the way, while it doesn’t change much about this more fundamental basis for my opinion that “standard English” is an offensive fiction, neither British nor American English actually have the backing of a nation state. This is in contrast to, for example, French, which does. According to this article on language regulators, “The English language has never had a formal regulator anywhere, outside of private productions such as the Oxford English Dictionary.” Prompting my rhetorical question to you earlier: Who is the governing body? There is none.
I would argue there is a distinction between the prestige markers you’re talking about and the more general grammatical rules that are followed. You can use grammatically correct English without subscribing to the over complexity that is, yes, possible.
A dialect can work locally, but there is a reason why many who have travelled abroad find themselves deliberately softening their dialect and shifting closer to proper English /queen’s English / whatever we’re calling it. It’s because their dialect is hard to understand for people who are not used to it. My girlfriend always comments how my dialect comes out of the woodwork when I’m back home with family.
Saying “I be lit” or “gimme a pint guvna” just will not be understood outside of the area or cultural group that it is spoken in. Therefore for business and/or travel you need a more standard form of English that everyone can understand. That’s not some overly complex cultural dance we play to keep the powerful powerful, that’s just basic requirements of communication.
“Should of” instead of “should have.”
“Me and her went” instead of “she and I went.”
“Flustrated” instead of “frustrated.”
“To who” instead of “to whom.”
“For all intensive purposes” instead of “for all intents and purposes.”
“Aks” instead of “ask.”
“Literally” to mean “figuratively.”
“Shoe-in” instead of “shoo-in.”
A semicolon instead of a colon.
Using a preposition at the end of a sentence.
Splitting infinitives.
Starting a sentence with a conjunction.
Each a simple “error” to remember. But there are thousands of them. None make an appreciable difference in understanding. None would ruin a business deal or a meeting except in terms of lost social standing for getting it “wrong.” This category of errors is what I believe to be meant by “improper English.” This is in contrast to “incomprehensible English.”
As I said, successful transmission of the message is the only true test of linguistic legitimacy. You’re absolutely right. People are instinctively aware of when their dialectical quirks are going to cause a problem communicating with outsiders, and they code switch. They simplify. Ironically, the less familiar the interlocutor is with English, the more “improper” a native speaker’s English might become. “My name? John. Your name?” Yet in so doing, they become more compensable because they’ve dropped the complex cultural dance which they are so often required by the powerful to perform.