We just had a small debate about this issue with other Kitware folks, and a potential solution that could satisfy everyone came up. What if vtkPointSet was not an abstract class anymore? It would make the class hierarchy almost untouched. You could then instantiate point sets without having to make it an unstructured grid or polydata and only have access to the wanted elements: points and their data. We would have to explicitly implement pure virtual methods related to cells, making them returning nullptr or zero when appropriate.
What do you you guys think about that?