There is value in linguistic standards - too often what passes as innovation or evolution is simply caused by laziness or ignorance. Stylistic departures from standards can work quite naturally in fictional narratives. In non-fiction, it's probably more important to have standards to adhere to.
I agree, there is definitely value in linguistic standards. I don't think we're
losing them so much as relaxing them, thanks to our increased exposure to people and groups and means of communication that we didn't have in previous generations.
In my Syntax class last quarter, I was shocked at how often we had disagreement as to whether sentences were and were not grammatical, and I mean that in the linguistic sense (is a valid utterance) rather than the prescriptive sense.
The worst offender in the degeneration of English is perhaps the business world, ultimately because of what they use it for - persuasion and obfuscation. Consequently, their influence it pushing language away from clarity and meaningfulness. Government follows a similar path, of course.
This is not new, though. Propaganda, whether it be business or governmental, has been around, and hasn't had a substantial change in our linguistic standards. People have been using the same style guides, with minor changes, for the past hundred or so years. It's only now they're whining that they're no longer useful. It's not the business or governmental tactics that have changed.. it's something else!
I do take issue with his comment that "to mangle the rules of grammar, you first have to know the rules" - that's patently false. Most people today make grammatical errors because they were never taught grammar at all, but picked it up piecemeal, and badly, from pop culture.
You're speaking prescriptively. We all, internally, know the "rules" of grammar, or else we'd never make ourselves understood!

I blame this on the education system, as you seem to, too. They haven't been able to make grammar an interesting topic, it fell out of fashion, and now is not taught, etc, but I also think there is a sociological aspect to it, too. People rebelling against the "rules" authority dictates, or the higher social class insists on. Ebonics is an extreme example of this, and although it may have grown out of a pidgin, it's continued use that transcends geographical boundaries is proof of its sociological basis. I actually like ebonics - it's interesting, quirky, and I don't at all blame it for the downfall of English. What I do blame is the <broken record> Internet </broken record> and the fact that we do so much of our communication in text form.
As a tech writer, I used to say "Writing is a privilege, not a right!" and by that I meant "Just because you're a native speaker and are able to express yourself through written word enough to be understood, you don't have the right to do professional copy because your language skills aren't up to snuff!" Same thing I'd say to a singer, or a bad public speaker. For some reason, it's easier to talk a bad singer out of singing, or a bad public speaker out of speaking, than it is to talk a bad writer out of writing. Do we, as a society, think our right to express ourselves in the written word is a right? Is it the lack of immediate feedback you get when writing vs speaking or singing? Dunno. Discuss!