My S applied this past year to many colleges, Duke among them. Duke was not high on his list, he had never visited, so he did not write the why Duke essay. I am happy to report that he was admitted to Duke anyway, visited on Blue Devil days and fell in love with everything Duke. He is now a very happy member of the Duke class of 2016!
So, the moral of the story is that Duke does in fact have a holistic approach and I believe is looking for specific students to fill their class, essays being just part of that approach.