Drysc rains on the parade, however, by saying that because Essences drop so much back in old Azeroth, it would be necessary (in his view) to nerf their droprate there. And that in turn, would cause problems for lower level players who couldn't make it to Outland (Drysc assumes that lower level players are still farming Essences the way all 60s used to, but I might disagree with him there). Also, he says, it would increase the amount of Motes and Primals floating around, obviously, and Blizzard doesn't want those to be super easy to come by.
But surely there's some conversion rate they could hit on which would make Essences worth just a little more than worthless at level 70. If too many Essences drop in Azeroth, then make it 15, or 20, or whatever. Better than vendoring stacks of Essences picked up while Mote-farming just because no one will buy them on the AH.