But then question is while would we have Decal node if there is no practical use for it.
DecalGroup there for now only for compatibility reasons, and Xith3D now exploits fallback statement from Java3D docs:
An implementation of Java 3D may fall back to treating DecalGroup node as an OrderedGroup node.
BTW, I personally was never using [or even trying to use] actual applications for DecalGroup, so if somebody will explain how it should work and give some example [say, with test code using Java3D] why this node needed, we can add its functionality.
Yuri