Actually, having read Ken's book, it was real obvious that he always made painstaking attempts to capture the best possible performances and sounding tracks that he possible could. In fact many of the artists would comment in the book that he would keep making them play take after take until he felt he had the best they could do. He also would spend a considerable amount of time getting the drums mic'd and sounding just right. In fact, several drummer quotes in the book said that they had never heard their drums sound that good in recordings, as when Ken tracked them.
However, Ken mentions several times in the book, that he fully understood that it was the song and the artist that would sell the record, not just his production. For example, the first time he was approached by America to record their album, he basically said that he was not interested, because he did n't think they had their chops together yet. There were several artist he names where he felt he did some of his best work, but the albums did not sell at all.
So in my own way of thinking, I think my answer would be that it is important to strive to be the best at what ever you do, be it musician or engineer. However, if weighted, the song and performance are still the most important part of the equation.