REST/Atom: Why do we have "TypesChildren" and "Types Descendants" collections?

In the CMIS domain model (Part I) and in the WS/SOAP binding, we said that there's a flat "types" collection returned by the getTypes method, and that the hierarchy of object types is not explicit (i.e. returned as a tree), but rather implicitly discoverable via the "ParentID" attribute of an object type.

However, in the REST/ATOM binding, there's a "TypesDescendants" collection that returns an actual hierarchy of types.

Why do we need/have this? If there's a use case for it, it should be part of the domain model & all bindings. If not, we should remove it from the REST/ATOM binding for consistency.