The grammar of a language consists of rules that govern how you arrange the morphemes, words, sentences, paragraphs, etc. that make up your utterances.
That said, why would I be hostile (and I am, if you’ve ever talked about it with me) to the idea that some utterances are “better” or more “correct” than others?
The problem is that “language” is a fuzzy concept. Language varies enormously between different communities, social groups, and all the gallimaufrey of human interaction.
Why do people persist in attributing value to one particular variety of language over another? As a tool of social dominance and status signalling. As Geoff Pullum notes in his excellent talk on the subject, the written expression of a particular form of English spoken in London a couple hundred years ago has become associated with, not to put too fine a point on it, being successful in business and politics. Therefore, using this one dialect, which is called Standard Formal English, merely out of a myriad of others in the continuum that is mutually-intelligible English, signals that you are a member of the upper class, that you had the priviledge and leisure in your childhood to become fluent in it.
Note that this post is written in pure SFE. I don’t wish to discourage the use of Standard Formal English, as it serves a useful purpose in facilitating communication around the world.
What I do intend to discourage is the notion that using other varieties of English is “wrong”, “bad” or “broken”. This is as ludicrous as the idea that wearing jeans and t-shirts is “wrong”. There are situations in which wearing jeans would be inappropriate (a funeral), just as there are times when wearing a formal suit would be inappropriate (the beach). Likewise, there are times when Standard English is appropriate (a job interview), and times when it doesn’t matter one bit (a text message to a friend).
To think otherwise is, in a nutshell, a morally reprehensible prejudice. I had a conversation the other day with someone who had believed all their life that “Low” German was a “funny”, primitive pidgin, thus consigning millions of people to the status of subhumans. I had some trouble convicing them that the “Low” and “High” in varieties of German referse to geography, not any qualitative judgement, and that the only reason that the “High” variety has the higher prestige is that it happened to be the variety that Luther spoke when he translated the Bible.
I will teach my daughter that Formal Standard English is a useful life skill, but I would never dream of telling her that she is a lesser sort of person if she uses abbreviations in a text message, any more than I would insist that she wear a formal business suit to the beach.