As universal server admin guide states:
The benefits of clustering include lower overhead (spreading the system load between the PGP Universal Servers in the cluster means greater throughput) and the ability for email services to continue working even if one of the servers in the cluster goes down.
Source, should have a read:
http://www.symantec.com/business/support/index?page=content&id=DOC4564&actp=search&viewlocale=en_US&searchid=1326228132893
This is also a good read:
http://www.symantec.com/business/support/index?page=content&id=TECH149975&actp=search&viewlocale=en_US&searchid=1326227450175