Nurbs are not efficient for realtime rendering.

There's no reason to encode all that curve data if all you care about in the end is a finite set of pixels rendered on the screen.

You can achieve the same level of detail without having to recalculate the surfaces. It's a lot easier for the computer to morph a shape by applying some math function (that's essentially arithmetic) to all the polygon's vertexes.

That said, I've seen some 3d artists work in NURBs, then render out to Polygons.