There is an extreme case of partition tolerance that must be considered: disconnected operation.
For users at the edge of the network, latency can be the biggest performance killer. If it takes 1 second or more for each user action to be reflected in application state due to round trip time (mobile web) those seconds add up and users can be frustrated.
However, if you move the database and web application to the mobile device itself, users no longer see network latency as part of the user experience critical path. Latency has been proven to be correlated directly to revenue, because users engage much more readily with snappy interfaces.
Once data is being operated on by the user on the local device, the key becomes synchronization. Asynchronous multi-master replication demands a different approach to consistency, than the traditional model which assumes the database is being run by a central service.
The MVCC document model is designed for synchronization. It's a different set of contraints than the relational model, but since it's such a highly constrained problem space it also admits of general solutions and protocols.
It's my belief that the MVCC document model is closer to the 80% solution for a large class of applications. Storing strongly typed and normalized representations of data is an artifact of our historically constrained computing resources, so it will always be a good way to optimize certain problems.
But for many human-scale data needs, schemaless documents are a very good fit. They optimize for the user, not the computer.