If you are not aware of Ebonics, African American English can be easily dismissed as a lazy degeneration of 'proper' English. But Black English is not in any way wrong, indolent, malformed, or uninformed--it's merely different from the English found in newspapers and blogs.
And African American English can in many cases express itself more elegantly than standard English. Toni Morrison described it as having five present tenses:
He writin. He is writing.
He be writin. He is usually writing.
He be steady writin. He is usually writing intensely.
He's bin writin. He wrote at some earlier point, but probably not now.
He's BEEN writin. He has been writing for a long time, and still is.
Anyone complaining that speakers of African American English cannot express themselves obviously cannot appreciate the wealth of expression afforded by these five present tenses. They also have probably never been forced to read literary criticism, and seen what it really means for people to be unable to express themselves.
I learned about these five present tenses from Slate's new language podast, awkwardly named Lexicon Valley. (What is the main agricultural product of Lexicon Valley? Semicolons?)
To return to the point. All spoken dialects--from the jargon of the nuclear engineer to the text messages of a petulant teen--differ to some degree from our more stolid, slow-to-change written dialect.
When we write, we are participating in conversations that began long before we were born. We step to the lectern that Swift and Twain not long ago lefy; we gaze at an audience who has listened to Salinger, Hemingway and Hawthorne. We speak to these dead luminaries, who, if heaven is fair stand backstage, listening patiently to those now speaking. And since we speak to the past in writing, our language is stiller, only sometimes referring to the bubble and bluster of our wild spoken Englishes.
And a bonus fact for you: What cohort of people has the most up-to-date English?
It's low-middle-class teenage girls. Yes. When you pass by a gaggle of texting crimp-haired gossip-flingers what you hear is the English that will be. At least according to the book Not By Genes Alone.