This technology was explored in another software called EIM, the ElectricImage Modeler. It was quite popular, though the company ended up canceling the software due to licensing problems with Spatial. They're currently planning to resurrect it.

What's nice about subD's within a NURBS framework is that you can sculpt using your familiar Polygon subD cage techniques, but also make use of NURBS boolean and trim functions (depending on how it's implemented). Also the final polygon mesh can be exceptionally clean, with density set by the meshing function.

Malcolm Thain was the acknowledged master of this 'sculpting' tool when using EIM.

http://transputer.org/3dgallery/main.php?g2_itemId=964

If this can make it into MoI it will really set the software head and shoulders above the crowd.