The best thing i can say is to use them all. Maya lets you perform even conversions from nurbs to anything else.
If your working on multiple platforms that may not support sub d, model in Nurbs and transfer it over.
If your working exclusivly in Maya, model in Nurbs and convert to sub D (or poly if low resolution is acceptable).
The clear benifit of nurbs is the modeling speed. If your really good at nurbs modeling, doing rough out shapes will take you no time at all. Compared to doing it strait in polygons, Nurbs with isoparm insertion takes as little as 20% of the time.
For example, model an eye in poly and model one in nurbs. with nurbs you automaticly follow an edge loop, with poly you have to make your own. with Nurbs your construction is automaticly uniform, with poly you need to split surface all the way around your model (unless you use 3 gons or 5gons, which are cumbersome when converting to sub D for Mental Ray).
Nurbs have definatly fallen a notch on the ladder but will never be off the ladder completely.
And after calming me down with some orange slices and some fetal spooning,
E.T. revealed to me his singular purpose.
--TOOL, 10,000 Days---