Since it's currently not possible to run in a mixed deployment with some replica set members having auth and some not, to enable auth for a set for the first time requires taking some downtime.

We could get around this by having a compatibility mode that authenticates all outgoing connections using the keyfile, but doesn't enforce any authentication rules on incoming requests. Then you could have a no-downtime switch by doing two rolling restarts of the set - one to provide the keyfile but start in compatibility mode, and another to remove compatibility mode and go into full authenticated mode proper.