I don't like this viewpoint at all. Spoken English and written English are different languages, full-stop. You can see this in effect when someone is giving a fully-scripted presentation or talk---it just doesn't sound like speaking, no matter how conversational the text is. This is in large part due to the issue of word choice: speakers must choose their words quickly, so they must transmit their idea using lots of small, common words, while writers have time to think about word choice and select the one that transmits the exact connotation the writer is going for. And that's not to mention the nonverbal cues which add a ton of richness to spoken English. Writing allows you to take time to look over your words which lets you re-gain that richness with poetic devices which really allow your words to flow in a way that would sound unnatural and forced in spoken language. I would go as far as to say that if your writing sounds good spoken, it is not very good writing at all.