The practical upshot is: On PC, I can take a single disk with a single GRUB version and boot it on an arbitrary machine. If ARM machines can do that with their own UEFI/ACPI/ServerReady setup, then great. But if I need to alter that disk for each machine I want to use it on (inject board.dtb, add a custom u-boot, whatever), then that's not okay. I would happily accept u-boot and whatever else being on an onboard flash chip that I could update but didn't need to care about otherwise - that would get us back to "your PC boots via a UEFI system that's a whole OS unto itself, but it chainloads GRUB via standard interface so who cares".