Major Section 11.1 states:
An entity container MAY provide a QualifiedName value for the Extends attribute. The value provided to the Extends attribute MUST resolve to an entity container in the entity model. All of the children in the extending entity container are added to the children of the extended entity container.
This seems to be the opposite of what http://msdn.microsoft.com/en-us/library/dd541078(v=prot.20).aspx states:
EntityContainer elements that extend another EntityContainer inherit all of the extended EntitySet, AssociationSet, and FunctionImport child elements from that EntityContainer.
The benefit of this feature is unclear; reuse mechanisms can and should be hidden within the service provider infrastructure and not shine through to the client.