Sasha Volokh argues that speech that's gramatically correct when addressing Ph.D.'s becomes incorrect when addressing inner-city youths, because essentially, they speak different languages. Yes, I've been in the situation where I'm talking to people who use a different variety of English, and sometimes I think I know what the "right" way to say something would be ("right" according to them), but I can't make myself say it. Apart from that, there's the danger of trying to say it their way and having it come out like a geezer using slang. Which in the case of younger people, it would be, in fact.