Enabling node-to-node SSL security is designed to prevent a specific avenue of attack. A node will not be allowed to join the cluster, unless it presents a valid SSL certificate in its Java KeyStore and the Java TrustStore matches with the other nodes in the cluster. When this level of security is active, it will encrypt communication between the nodes over port 7001.
Node-to-node SSL may seem unnecessary, but without it, an internal attacker could join a rogue node to your cluster. Then once it has bootstrapped, it will shut the node down and its data directories will contain at least some of your data. If you don't enable node-to-node SSL, this can be done without having to know any of the admin passwords to authenticate ...
