For whatever reason, NoS group content is very low on the group damage side of things (minimal AEs or single target damage effects on characters other than the tank). This leads to a design that is more advantageous to a tank that can self heal since, in theory, a group could be formed with no healer at all. Going back to an expansion like TBL, which was rife with AEs in group content, and even merc healers were not up to the task of keeping the group and the tank alive. It's simply a shift in content design. Content can be (and has been) designed in such a way that there is no functional difference between the tanks in group content, insofar as they all require the same support.