Mailing list discussion:
The main use case for this is to allow the publishing of the same feature type in different virtual services (i.e. different workspaces), this is particularity important for complex features (app-schema).
Sometimes we need to map the same complex feature type (app-schema) in a different way and publish it in different end points (virtual services). Because a feature type is associated with a specific namespace and namespaces have a one to one relationship with workspaces, the only way to this right now is to use several GeoServer instances.