Returning the current setVersion, electionId and lastOp would allow drivers to avoid pinging the server using ismaster continuously to establish the topology of the replicaset as well as provide monotonic read operations.
In case of a primary election a new electionId would be returned and in the case of a new secondary being added or removed a new setVersion would be returned.
If adding a new secondary that caused an election the server might return both a new setVersion and a new electionId.
the lastOp field would allow for simpler monotonic read operations