After spending ten years as a micro publisher, allow me to state the following:
1) Non-fiction should be written using "book English," meaning it should be as clean, clear, grammatically, syntactically, and lexically correct as possible because the purpose of the book is principally to convey information. Clarity is king.
2) When it comes to fiction, should you have a publisher that insists their way is correct and won't talk to you about it, find another publisher. Fiction is more about emotion than it is conveyance of information.
An excellent example (though non-English) is the Finnish book Tuntematon Sotilas (The Unknown Soldier). Written Finnish is a notoriously grammar-heavy language, but the spoken language has dialects just like English. The author (Väinö Linna) chose to "write" the dialects. Finnish is phonetic, so it wasn't hard to write the dialects --- but it's painful to read! That, of course, was part of the point. The author wanted his dialog to be culturally accurate, which meant breaking nearly every rule in one of the most grammatically structured languages on Earth.
Tell your publisher that you're willing to work with their editors to make a superior book --- and then do so (please!). Take advantage of their expertise and remember they have a lot of experience building marketable books. But contractually reserve the final decision to yourself. If they won't do this for a work of fiction, find another publisher. Remember, they're a business, you're an artist, both needs must be met or the book will have a tough mountain to climb for success.