A SessionPool should be associated with a Cluster, not a Client. This way, new Clients created from existing ones using #with whose options do not require a new Cluster don't create their own pools.
There is no reason for Clients with the same Clusters to have different SessionPools.

Note: This is necessary for Mongoid's use of sessions. Different models sometimes have different clients that have the same cluster, no reason to have different session pools for these models.