Right, I like to think that there are milieus of English with which knowledge and conformance can be useful. Formal writing for example. But to say that the rules of formal writing should be extended to all domains is akin to saying that all food should be stir-fried. Just like there is no such thing as standard cooking, there is no such thing as standard English.
The preferred euphemism used to be "non-standard", which still leaves the problem of defining "standard" English.