理由はこちらにありました。
https://forums.oracle.com/message/9692198#9692198
If you are using a crossover cable and you shutdown node (A) you will loose the network (private) link from node (B) (this happens in some servers.), Oracle RAC will not work with private network link down, both nodes will down and will not start until you get link on network private. (Goodbye high availability)