By making threat gen stats also generate resources that are used to actively mitigate incoming damage, the goal is to make tanks want those stats, rather than simply aiming as close to complete coverage of the combat table as they can get, reducing incoming damage to something as reliable and easily anticipated by healers as possible. Tanks currently value dodge, parry, and their mastery stats well over any potential threat generation from hit and expertise.
Since we've already seen quite a bit of the Mists of Pandaria talent calculator, we know that design of the new tanking system is probably fairly well advanced. We also know that the monk, another tank/DPS/healing hybrid class, will be debuting with the expansion. Therefore, it's worthwhile to examine tanking changes that could be implemented, even to stretch our vision of tanking significantly past where it is now and most likely past where it will go in Mists.