If WS-Addressing message information headers are used in this way in specifications, we can't define a message which can carry multiple subscription IDs; in the case, for example, one wanted to cancel multiple subscriptions with one message:
Batch operations for network requests are a critical element of performance which enhances scalability - at least within a single organization. A REST based approach with explicit support would be fairly easy to outline, but I'm thinking it might be easier to model this as pipelined requests. Not sure. If anybody has any thoughts, drop me a line.