leenia -

Not everyone speaks or writes in standard English. In fact, most of us don't. Unfamiliar dialects may be difficult to understand, at first, but it was a real breakthrough when I read Zora Neale Hurston who wrote in an African American dialect. After reading her, I went on to read other African American authors and enjoyed the colour and the texture of the language.

If you can't write the way the characters actually speak, it sounds false and isn't it the objective of a good writer to speak the truth? Besides that, the page is an empty canvas and if you write in a language that is not your own, the words become white on white. Standard English can be quite boring and contrived.