Why would Apple underclock the graphics card?
Presumably temperature and/or battery life.
I assume it would lower heat emission and energy consumption, so battery life and an aluminum body that's cooler to the touch would be the reason?
Probably. Could be a reaction to the 8600s frying so many MBPs a couple of revs back, or it could be just general. If it's because of the overheating thing, that makes it even more annoying that they didn't use a lower-wattage, but faster, GPU.