GT, the term I prefer to Standard English is "Received" English. We aren't talking accents here at all. One can speak in a Cockney or Piney Woods accent and be perfectly correct grammatically (and omit filler words such as "you know"). English is very fluid in terms of vocabulary, but its grammar really hasn't been. Like Win (sorry, as) Win pointed out, if you read the literature of 100 years ago or 300 years ago, the basic structure of the language is pretty intact. The sentences literate 19th-century people (the letters that came out of the Civil War are extraordinary) are sometimes much longer than ones written now, but not because the grammar was different. People simply used those grammatical constructions more then. I think some of the decline in English grammar has come from the mindset of "get the facts out fast" that is often found in business. The argument that if one is understood then everything is fine goes only so far. Ultimately, it leads to a lot more misunderstanding. So much of speaking and writing is simply habit. Grammatical rules are generally not difficult. They just require attention until learned.

(BTW, I watched Mark Ingram give a 10 minute interview and say "you know" only once.)