2. I was under the impression that almost all Mongo drivers had connection pools.
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.