A Service often needs to provide additional data, such as service name, organization, model version, .... This can be defined in an entity type referenced by a Singleton in the service document, but there really is no need to define a separate type for this purpose or for the additional complexity.
Currently the Service document (EntityContainer) is restricted to two specially defined property meta types: Singleton and EntitySet. Assuming null for Singleton is allowed, ODATA-1183, the functionality of these types is equivalent to functionality provided by containment Navigation Properties.