I re-released my entry on availability, logical failure, and vendor diversity. In the article I explain why running two identical devices in a hot swap configuration won't protect admins from software or logical system failures.
Martin Fowler called this problem catastrophic failover, which basically means if there is a logical error in a system, failing over to another identical system will cause the backup system to also fail. I experienced this first hand with a redundant system I worked on. A software bug propagated and cleanly brought down 24 servers. Not fun.