Well in theory the only advantage I see to subd over smooth preview is localization of detail.

In subd you can select an isolated region and further subdivide that region and add more detail.

Creasing could arguably be considered an advantage but polygons now have edge creasing, so. Also, Maya subd creasing is sub-standard (pun intended), in that you only get to choose full and partial.

However, in both of the above cases all your modification are lost if you convert back to polygons, while in other subd packages like modo, blender, and silo this is not the case. For this reason alone I find Maya subd's unacceptable. Working alone it's bad enough when you are forced to convert back to poly and lose all your subd edits, but in a collaborative environment it would be a disaster! Consider a model several steps up the pipeline and then someone converts the model back to poly and all the subd edits get wiped!