Inside raids yea, self-healing doesn't matter in raids anyway because there are dedicated healers to heal you, but outside of that Mages are one of the worst classes.
Almost all non-raid content is solo content, and that's where WoD Mages are unviable, especially during the first patch, when most mobs are hitting like trucks.
No self-healing combined with no burst capability (removal of Evocation healing and AT's buff snapshotting) is gonna kill the ability to play Mages outside raids and dungeons. Timeless Isle with a 496 geared Mage will become impossible in WoD, unless you group with a dedicated healer.