SSH: passwords or keys?

Again, it is not always possible to have humans bash things in to all
systems that run unattended and have to connect to other systems. For that
subset, nonpassphrased keys are reasonable. (For the application I'm
thinking of, if they steal the drive we silently fail over, and, ooh, the
attackers would be able to run a backup without our knowledge! How
terrible! Of course, if they've stolen the drive, they're going to be on
the wrong side of a firewall anyway. This isn't *my* private SSH key: this
is a key created specifically to allow a single backup daemon to stream
backups to the backup server. That's all.)