Leolaia,
I dont think we are talking about black english v white english. Everyone may drop their speech to a more casual level when they are at home as opposed to in the professional world however I think what Bill Cosby was talking about people who are so uneducated that they do not even know the proper pronounciation of words so as to use them in the world. Its surprising the number of people who use phrases like 'where you at?' 'dat aint mine' ' dem's his kids' ect ect. That is NOT black english or any kind of black dialect. Its simply poor grammar as a result of lack of education or lack of desire.
That is not to say that there isnt a 'black' version of the english language. As a professional I spend 90 percent of my day speaking to clients on the phone and 90 percent of the time I know when I am speaking with a black person and they know that I am black. I have a very professional phone manner as do they and thus it has nothing to do with poor pronounciation and misuse of words that gives our speech its cultural distinction.
It is a mistake for people to keep trying year after year to give people who speak poorly an excuse for their lack of education. Ebonics, Black Vanecular English,.....whatever. Its all just laziness. Those people, black, white, hispanic or whatever, would just rather spend their energy complaining rather than opening a book and learning something. They are taught from infancy to sit around and watch tv instead of reading books or to worry about what kind of clothes they have on rather than what kind of grades they make. Their brains are so unused that it becomes hard work to learn the simplest thing.