I've always felt that any person should feel free to speak anyway he wishes with his freinds or in the neighborhood. Nevertheless, in a professional or customer service situation WE ALL MUST BE UNDERSTANDABLE TO OTHERS! Malcolm X, for example, was able to speak English better than most radio broadcasters when speaking to the media.
Interesting side note: My sister told me that the blacks in South Carolina where she now lives are more understandable than the whites.