Let's face it, writing what you know is only short-sighting your writing career.
Sure, John Grisham was a lawyer, so it seems natural that most of his novels are in the legal thriller arena. But not all of them.
Stephen King was, at one time, an English teacher. Many of his characters are also. But not all.
Anne Rice spends many of her weekends hanging around vampires and swapping stories--okay, that one's a little far-fetched, but you get what I mean.
I'm almost positive John Sanford--who is a pseudonym, besides--never worked as a hard-charging police investigator.
What about J. R. R. Tolkien, J. K. Rowling, Robert Jordan, and Brandon Sanderson? Their fiction is largely in the fantasy world. Can't use any of their "write what you know" there.
I think it should be rephrased to be "Write what you want to know" or even better yet is "Write what you love." There's nothing with writing what you love to read. In fact, it's strongly encouraged.