I am using ESXi 5.5. Here's the scenario I had in my head:
I connect remotely via DRAC (Dell remote access), or HP ILO. I patch ESXi, reboot it, and the server doesn't come back up. Maybe it's a PSOD. Maybe worse. From what I've read, these little USB drives do fail. I keep a bootable ESXi install CD in the drive of my hosts. Using my DRAC, I can specify to boot from the CD, and then do a new install & patch of ESXi to the second USB drive. Takes about 30 min, bring it up, either restore my config or re-add my VMs, and I'm back online.
That was the idea, anyway. I don't have clusters, HA, etc.