The answer to the first question is that Swing widgets, in spite of what Sun's marketing machine says, are large and slow. Figures are small and (given the underlying platform) reasonable fast. We're interested in creating components with zillions of Figures on them, so we took some pains to keep them small.
The answer to the second question is that we don't really know. We wrote PaneWrapper as a first attempt at exploring nested "surfaces," and have been wanting to figure out to integrate them with Swing properly, but it just hasn't been a priority so far. If you have made custom Swing widgets with Java2D, email us some code.
So, maybe later it would be cool to try layering Java2D over Java3D and extending the Diva look into the third dimension, but for now we don't really care that much :-)