I am white, and I grew up in a half middle-class, half working-class white milieu in which Black vernacular was snickered at, made fun of, and looked down upon. I've since encountered this attitude in too many (white) contexts to count.
The gist of this typical racist attitude is as follows. Black vernacular is an "imprecise" way of speaking and expressing oneself. It is a less clear, less "correct" way of talking that flaunts (or evinces ignorance of) the timeless laws of grammar and syntax observed by "civilized" whites. Moreover, black vernacular signifies the "low", the uneducated, a lack of tact and "civility" and so forth. Finally, insofar as black vernacular is meaningfully black at all, it is taken to be bereft of genuine value and accordingly deserving of no respect (because, of course, things only have value at all insofar as they borrow from or give a nod to that which is marked out as "white"). "If they were more only more educated, they would just learn to speak like us".
Now, this is hardly a peculiarly American phenomenon. Accents, dialects, and other modes of expression have long served as markers for various sorts of social and political distinctions. The tight connection between accent and class in Britain, for instance, has long been noted. But the particulars of the case of Black vernacular in the United States are, like any such asymmetry of power that is partially expressed in cultural spheres, a product of the precise history and politics of race in America.
Before tearing the above racist attitude to shreds, I'd like to note a paradox at the heart of this white scorn. First off, it is plainly obvious that black culture, and black vernacular in particular, has for some time been a fresh source of material for mainstream white culture to appropriate and gentrify. Words that are by now commonplace in many different white contexts (e.g. "cool") are examples of how this process functions. The paradox I'm trying to bring to light here is this. That which is taken to be white is constantly appropriating, colonizing and redeploying various gestures, styles, and modes of expression found in black culture. But at the same time there is this inbuilt disdain for blackness, a patronizing attitude that follows the contours sketched above. But this paradox shouldn't be taken to be a strange side-effect or problem of interpretation here. A properly dialectical analysis brings contradictions to light rather than explaining them away. One such contradiction is the fact that whiteness itself comes into being and defines itself against that which is non-white, paradigmatically (but not entirely) that which is black.
But let us return to the set of attitudes expressed above. Is there no truth to what is said there? I want to say that the entire way of thinking about black speech expressed in this set of attitudes is misleading and ideological (in the pejorative sense). First off, it is just false that black vernacular is a somehow less clear, less expressive or less precise than other more conventionally "white" modes of expression. In fact, sometimes expressions in black vernacular can be more efficient and effortless ways of expressing certain thoughts.
Now, there may be some truth to the claim that it is "less clear", but that could only mean "less clear to those unfamiliar with it as a mode of expression". And that is true of any mode of expression whatsoever. I am often baffled by various Australian turns of phrase or colloquialisms, but it's not as though I take that to be a problem with such modes of expression themselves. So it can't be a problem for black vernacular as such that those unfamiliar with it are less able to grasp it easily and quickly.
Second, to say that black vernacular flaunts the "timeless laws of grammar" is bullshit. There are no timeless laws of grammar. Grammar, syntax and other features of our modes of expression have always changed over time and have always been sensitive to various dimensions of the present situation (whatever that is). Modes of expression are a moving target, and though they shape our sense of what the social world is like, they are also shaped by the way the social world is. It is not an exaggeration to say that the vast material reconfigurations that followed the emergence of industrial capitalism inaugurated an entirely new set of concepts and expressions (e.g. "revolution", "industry", "capital", "consumer", "wage labor", "globalization", etc. etc.) needed to make sense of that radically new conjuncture. Today, texting, social media and so on are inflecting and changing our modes of expression as well.
So grammar is always in flux and our "sense" of what is grammatical and what is not is constantly changing as a result of new uses and other linguistic innovations. To say that black vernacular doesn't respect a timeless, embalmed view of how wealthy whites were supposed to express themselves in the 1950s, say, is not a knock against black vernacular. Who is it that cares about preserving certain contingent linguistic tropes from ruling class white ideology anyway?
So all that's left of the complaint against black vernacular is the claim that it's "deviant" or "low" insofar as it doesn't express how an upstanding white (ruling class) conformist should think and express herself. But that's no complaint at all. That just is a certain form of racism, viz. the systematic and irrational devaluation of all things deemed black (because they are black). That this kind of racist devaluation is bullshit is even something that white racists seem forced to concede, since they themselves make frequent use of "black" expressions, cultural norms and practices, etc. Southern culture itself, however much the racist ideology of Jim Crow tried to deny it, is partly constituted by non-white cultural norms and practices. Elvis, Johnny Cash and the Rare Earth are nothing if not gentrified expositors of a tradition that was born in the black community. The whole structure of conventional "whiteness" falls apart if you ignore the contributions of black culture and practices that constitute it. A more consistent version of this devaluing, anti-black attitude would force many whites to abandon cherished practices and cultural allegiances that constitute who they are. Their own practice of systematically devaluing that which is marked out as "black" is, in fact, inconsistent with their basic mode of being.