Having something good, creative or worthwhile to say and saying it well are two different things. I understand the phrse "good writer" to mean the latter.
Too often have I come across excellent insights buried like diamonds in a dungheap in ugly, tortuous prose. This runs the great risk of these insights being lost or misinterpreted.
The converse is possible too, of course; utter nonsense put beautifully.
To support that, ghostwriters and copywriters are two examples of writers that take the gist of another person's thoughts and express them well in writing. They are creative writers, but they don't necessarily have to have anything to say to do their job.
