It would be interesting to know the percentage of Ph.D.'s who continue publishing or writing, beyond completion of their dissertation. For example, the percentage of Ph.D.'s who earn the degree, use the degree in academia/business/ministry/etc., but never publish a book or write articles for journals.
I would say it would depend on what the person did wiht the degree. If they went into academia, some schools require publishings, if consulting, I would say the same for marketing reasons.