To produce various visual effects in Swing,
a container needs to be notified when any of it children is repainted.
With the current API you can only achieve it by replacing the system RepaintManager
which leads to some known but annoying problems

http://weblogs.java.net/blog/2008/04/13/repaintmanagers-side-effect

The new JLayer component does have this feature,
however it makes sense to open the door for the existing components
like JXPanel from the SwingX library.

With the proposed feature JXPanel will be able to support the tranclucency effects
without setting their own RepaintManager.