The subsystem IDs roughly identify the PCB a component sits on. So unless you have multiple pluggable boards in a system there should only be one subsystem vendor and one subsystem device id.
I agree with Eric here. I question the need for subsystem IDs in the first place. Is there a particular OS that requires this info???
Yes, Linux for example, for certain devices.
I agree subsystem IDs are useless in most cases. But for certain devices there is the established practice to mark certain hardware configurations (or device firmware type/version) with these IDs, so coreboot will have to follow.
Segher