Right, I don't use it anywhere else. You could probably add a string description of it somewhere into the PCI driver (for /proc/pci's use), but you don't need that to get it to work. The reason it's there is just for symmetry more than anything else -- most of the other chipset device ID's have a _0 and _1 variant (usually for the north bridge itself and the AGP bridge sub-device-thingy), so I thought it might be good for this one to have it as well. If you don't want it, you can probably leave it out, although /proc/pci will complain that it's unrecognized then. Although it may complain anyway, I don't know.
There's a superficial reason for putting it there, though -- to keep the device IDs that the kernel knows about in sync with what you know about. But other than that, it's not needed.