I was told that the SCSI mode page commands that provide serial numbers are an optional feature. Is this true?
Grub2 most definitely fails to function on disks that do not provide serial numbers. See LP: #604335 for details. The key issue is that while the /usr/bin/grub-install script does not look at /dev/disk/by-id/* files, the code that running "sudo dpkg-reconfigure grub-pc" executes looks for only disks that provide serial numbers by using the contents of /dev/disk/by-id/* as the source of information.
One of the critical aspects of this is that VMware Fusion /dev/sda emulated disks fail to provide serial numbers (comment #44 in LP: #604335 above) so they fail with grub-pc.
How many non-virtual disks do not implement serial numbers and therefore cannot be used with grub2 correctly?