Hooey. The discussion there is whether grammar (or a dictionary) is descriptive or prescriptive. Most dictionaries fall in the former category, at least in English. They tell what is in current use.
A grammar book, on the other hand, is the latter. It tells us what is established correct usage. Why? Because we want those who are not necessarily part of our local or regional culture to be able to decipher what it is we're writing about. Why teach grammar, unless there are accepted rules?
So, I agree with H. Beam Piper: Many of the ills of our modern society can be traced back to grammar teachers who let incorrect grammar and spelling slide.