> Why would you be rebalancing under heavy write load. Wouldn't it be scheduled for quieter periods ?

I think the example case is when a node fails during a load period - unplanned vs planned.

Some of the public MongoDB stories come from shops that were forced to rebalance during peak loads because they failed to plan to do so in a maintenance window.

Of course waiting to scale out until _after_ it becomes a need will result in pain (of various degrees) no matter what the data storage platform.