Polys allow better control over details...its incredibly difficult to get good detail when u can only work with control vertices...especially for organic models.
Nurbs help get the general shape and proprortions and keep the model smooth, then you convert to polygons to add detail easier, and then into sub-d if you like to help refine the model even more.

oh, working in nurbs first, also eliminates the amount of vertices that need to be manipulated. You can get a pretty good general shape by manipulating just a few control vertices, as opposed to a lot of polygon vertices.