But in general, modeling solid 3D objects, which shall be fabricated, the paradigm of choice is bounded geometry and, for mastering the implementation complexity, NURBS are an appropriate unified representation.
In my factory automation time, my team had to deal with such requirements daily, suffering from a Babylonian variety of descriptions, representations and implementations.
With this background it is great o see, that Mathematica 7 offers Comprehensive Spline Support .
Geometric modelers, CAD/CAM systems have this all?
This is correct, but in black-boxes, you have to rust on their accuracy and robustness, or not.
Doing principal design in Mathematica you can get insight, with symbolic handling and in combination with other mathematical settings, dynamic and interactive manipulation, ... Bezier, B-Spline, NURBS, .. "friends" coming back to me in new clothes.