Comments
One negative comment: FHIR should not be specifying resources that exist solely to represent value set definitions. By this I mean there is no need to define the content of an externally available value set using FHIR resources. By providing this function, FHIR must then support resources to create Value Sets that are fully compliant with the VSD DSTU. This is not a function that requires or is best served by FHIR. FHIR resources should require all necessary elements to fully specify a single value set expansion that meets the need of the implementation.

Grahame's Comments
Implementers need to be able to share and reference value sets as resources. They are a key element of Profiles, Questionnaires, DataElements and other resources. Furthermore, the FHIR restful interface provides an easy mechanism for implementations to subscribe to, search for, retrieve, expand and otherwise manipulate value sets. Whether a given value set is "externally available" has no bearing on whether a resource should exist.