Network partitions are extremeley rare only for small clusters. For very large clusters or multi-datacenter clusters there is much more hardware than a single switch between servers. Then, the likelihood that something cuts off the whole room full of servers from the rest of your cluster is not something safe to neglect.
http://elasticsearch-users.115913.n3.nabble.com/CAP-theorem-...
Not only are network partitions rare, the most disastrous case where the cluster splits in half is even rarer. Usually, you have a small part of the cluster partition away.
I hope people don't take this as a Dynamo vs. Couch discussion, because the relative importance of partition tolerance is a topic that spans all datastores that give up on ACID.