Right off the bat, I'm sure many developers don't use datums.
However, including a tree displaying datums like Dream Maker does for atoms in map editing mode would be useful in organizing games that rely heavily on datums for managing information. In the map editor, you can click on the atom tree and double click an area/turf/obj/mob to go to that atom's entry in a .dm file. This would be great to have for user defined datums as well.
Alternatively, it works as a class manager showing and linking to any class the developer has defined (datums, the standard atom types, custom atoms that can't be added to the map, etc.).