Is there an obvious reason this isn't done?
Yeah - the filer's data structures don't keep information around in such a fashion as to allow us to refer to a disk by any name other than disk ID number by the time it's been unloaded, but disk ID numbers are kind of a gross way of referring to disks (they have no correlation with anything other than the order in which the filer saw the disks, and can change over a reboot).
We plan to fix this in a future release. The plan is to use the HA.SCSI# ("HA" means "host adapter", "8a" being the name of the first SCSI adapter in card slot 8) names universally.