Mathematically, SDS are a generalization of NURBS and do not differ that much implementation-wise (that's why apps like Maya can convert NURBS to SDS). The current version of Pixels3D is very NURBS-centric, which is probably why they haven't implemented SDS yet, but rumors/hopes/speculations/hints are that this will change in future versions. Also, Tempest does have the same "smooth polygons" function as FireFly, which is replacing polygons with B-Spline patches that follow the Phong normals. They're not as smooth as SDS are, but they work quite well with existing meshes. A change to SDS or NURBS will, however, break with a few things that Poser users became familiar with, at least with Tempest and 3Delight: The RenderMan interface, which they both use, does not allow to split a SDS or NURBS primitive in material groups.