When editing or proofreading an author's work, I always draw a distinction between dialogue and narrative. Dialogue should suit the character(s). An authentic accent, vocabulary, and pace of speaking is always appropriate. This should not be carried to an extreme which makes it difficult for the reader. I do have a problem with a character whose grammar is very poor to the point that it detracts from the story because it is so hard to read.
Narrative, on the other hand, should be closer to standard English (as fits the author's locale). I see no problem with dialogue being distinctly different from the narrative. Obviously an exception should be made for a story told in the first person; the narrative could reasonably be as nonstandard as the dialogue.
For me, it's just that simple.