That seems a strange sort of logic to me. Haven't compared the ground clearance between the live axle and IRS versions, but I'm pretty sure your clearance in back either way won't be worse than the front, so what difference would it make?
Note that Land Rover has a very similar independent rear suspension design on their rigs. I owned a 2007 LR3 with this setup after having owned a 2004 Discovery 2 with the solid rear axle setup. The rear suspension arms hung low like those on the new Yukons, and everyone in the Land Rover world was worried about them, but their fears were proven to be for naught. The LR3 was a far superior off-road truck to the Discovery 2 in every way. Mine was bone stock and it made it through the infamous "Metal Masher" in Moab with no issues. I don't think the IRS in these new trucks will cause any trouble at all off-road.
Has anyone experienced/heard of any problems from the control arm's low clearance? I don't do any rock-crawling but driving on dirt/rocky roads (nothing extreme) or through snow are my main concerns. Seems like anything deeper than 4-6 inches will have the arm plowing underneath the truck.