For one thing, the idea that there is only one right way of doing English – and everyone else is doing it wrong – is inherently flawed. And by “flawed” I mean illogical, elitist and even oppressive. Judgements about what counts as “right”, “good” and “correct” in writing and grammar always – ALWAYS – align with characteristics of the dialects spoken by privileged, mostly wealthy, mostly white people. We make these judgements based on learned biases, as well as a certain emotional attachment to our own way of doing things. But when people study dialects in an objective, scientific way (which is what cunning linguists actually do), they find that low-prestige dialects, such as African-American Vernacular English or Cockney English, have fully-formed grammar rules of their own that make just as much sense as any others. They are perfectly valid and functional forms of communication used by millions of people. The only difference is that they don’t have people running around telling everyone else to do it their way.
This is well worth a read, and is describes one of the reasons I hate the term “Grammar Nazi.”