I fail to see its value. What purpose does it serve?
The only 'useful' place is in the run once the boot order - as we only
define as the boot sequence 'Hard Disk' without mentioning which disk.
We can clearly replace 'Hard Disk' with the list of all disks (and let
the guest decide) and deprecate that flag ASAP, or we can have the list
of disks in that list (and deprecate that flag).