I never really got into using sub-ds especially when poly proxy already existed and CPS tools before that. A few setting changes and you could work on the proxy cage side by side with the smoothed mesh and have everything on different layers to avoid bogging down the machine. KB used this method in his Apache tutorial. Now with the smooth mesh preview, you don't even have to do that, though it is still kind of nice as a teaching method when you can see how your splits affect something without toggling back and forth.