Your last sentence is the answer I’ve always heard given: this in the context of a performance trade off — do you wait for every node in the system to acknowledge an update and purge caches, or return as soon as a sufficient durability threshold has been hit?

At the benefit of added availability? absolutely. This is the entire premise of mongodb.

No. A magical do-everything datastore would have strong consistency. Eventual consistency is something you might settle for given the performance cost trade offs.

To see this, consider that strong consistency has all the guarantees of eventual consistency, plus some more. And the additional guarantees might make your application much easier to write.

Job security?

Train tunnels

No.