If the machine is automatically rebooted and unattended (as is true of a
lot of unattended systems with panic_on_oops and reboot-on-panic set), you
really can't rely on having a human present to manually type in a
passphrase when the system boots. The sorts of keys I'm thinking of aren't
superprivileged anyway: this isn't 'ssh to root' but 'ssh to my
counterpart on some other system to exchange data' or 'ssh to the backup
server using an ssh subsystem designed for one-way backup transfer' (i.e.
you can't use it to *steal* backups). (And yes, I do both routinely.)