But that seems pointless. Exactly-once covering a subset of the pipeline doesn't guarantee exactly-once. At best it improves your chance of exactly-once deliver

But that seems pointless. Exactly-once covering a subset of the pipeline doesn't guarantee exactly-once. At best it improves your chance of exactly-once delivery. If you really need exactly-once delivery you need to make it end-to-end. If you don't support end-to-end exactly-once than you are really just an at-least-once delivery system. So you are making a guarantee that provides little if any benefits because it can not be tied into the whole pipeline.