When a user asks about setting up geographically diverse replicasets, we recommend using an odd number of data centers so that they can keep a majority live even after one fails. Obviously, for some customers that's not an option, but for 99% of them, sticking an arbiter up in the cloud is actually feasible and is the best answer to their problems.

Our docs, however, recommend putting a majority of nodes in a single location, thereby guaranteeing that if that location goes offline, that their replicaset will be read-only (or if it's a sharded environment, completely unusable). Mentioning that as an option for users who absolutely, positively can't have a 3rd location is fine, but it shouldn't be the default recommendation.

Doc is here:
http://docs.mongodb.org/manual/tutorial/deploy-geographically-distributed-replica-set/

Thanks,
Nicholas