Producing these ornamental designs as polygon models can be more time efficient, and give you a more fluid result, than what you would get with nurbs.

One advantage of using nurbs in this situation is if you wanted to reuse the models at a variety of sizes.

If you model in polys and discover later that you need to reproduce your model much larger, you would then have to increase the number of polys in your model so you don't see them in the finished part. While this can be done fairly quickly and with good results in many poly modeling applications, at some point the size increase might require rebuilding the model.

Another advantage of nurbs is that boolean functions like joining or subtraction typically don't cause issues for 3d printing. However, these functions can break solids in a poly modeler, so they might need to be repaired in a specialized program so they can be printed successfully.