Currently, MULTI_PING is needed to forward discovery requests to multiple discovery protocols. If we change the API in 5.0, and pass a Responses object down to the discovery protocols, and each discovery protocol forwards discovery requests down (and responses up), and they all add responses to the same Responses object, then we would not need MULTI_PING any longer.
However, this is an API change, so can be done only in 5.0.