I agree with that. Good writing should sound like spoken language. One of the classic mistakes of beginning writers is to use excessively formal diction, e.g. to use connectives like "furthermore" that they'd never use when speaking.
If you don't, then many other kind of glyphs and grammars arise. At the extreme, diagrams, mathematical expressions, etc, often lack even a correspondence with spoken language. Similarly, spoken languages have many subtle indicators and markers (often temporal or intonation-based) that isn't easily translatable to the written word.