Right now, many sorts of operational scenarios for working around bugs or limitations or operator errors require restarting replica set members without --replSet in order to be able to write some collections. This is kind of cumbersome for normal people. If some kind of online maintenance mode could allow users to write to the replicating secondary (possibly pausing replication for the duration of the maintenance), that would make these scenarios easier for people to work with.

Things I'm thinking of:

• resizing the oplog
• fixing a replica set configuration if the member doesn't like it and goes into a bad state
• (maybe) building indexes on secondaries, so that not all secondaries in a set get their index build commands at the same time