Note that by disconnecting all network interfaces simultaneously you are simulating "Split-brain", or network partition, which is a condition that you should never allow your cluster to go into. VCS uses the term "concurrency violation" for any application that has gotten into this state of running simultaneously on more than one machine.
All clustering products hope to never get into this situation because it means that you might have possible data corruption, if the application is allowed to run on both sides of the cluster simultaneously.
Some clustering products use Quorum disks to help prevent this, however usually Quorum disks themselves become points of failure in the cluster, and usually cause more downtime then they actually prevent.
Some clustering products use disk heartbeats, but again, depending on how this is implemented it could also cause more downtown due to disk failure than it actually resolves.
Also both Quorum and disk heartbeats don't prevent certain types of split-brain, like those caused by intermitten system hangs (or on Solaris: "Stop-A" followed by "go" a minute later).
The only feature I've seen that absolutely prevents split-brain in all these conditions is I/O Fencing, which is a VCS feature, but only on UNIX platforms, and only if you have modern disk arrays that support SCSI-3 Persistent Reservations.
I recommend having as many heartbeats as possible, specifically having low priority LLT (VCS) heartbeats on the public interface. The chance that all your private and public networks fail simultaneously (but the fiber cables and storage remain) is next to zero.
In my opinion: public low-priority hearbeats are the simplest thing you could do, involving the least amount of work and risk, that cause the greatest benefit in preventing split-brain.
