One of the main advantages of NURBS is that they are teselated(transformed into polygons) at rendering time, according to camera distance. That means that if the model is further away form the camera it will have less poly count and therefore will be faster to render. In a close up on the other hand it will have all the detail you need.
With polys it is always the same no matter the distance from the camera.
I have seen some excelent models of say cars and planes with polygons but the poly count is amzingly high.
But of course all three methods have their advantages.