Hi chris D

Its to do with tolerance when parts must fit together. Most CNC software has limited cutting control on cutting poly models . Its ok for organic and rough shapes , but other "machine part" shapes are better modelled in nurbs.

All curves ,circles and arcs are faceted in poly modellers. This is a major problem . Its still possible to machine, but you end up remaking some of model in nurbs or adding machining curves . Its a pain.

Nurbs is always better for manufacture and accurate work. I use 4 different CAM software depending on job.