Annihilation selfheals is rather small by itself, but the real kicker is the legendary utility Thirst for rage, while it only has small ticks (1324 per tick in BiS gear), you need to remember you get that tick everytime you use a rage consuming ability.
Combining anni selfheals from tree with Thirst for rage gets you almost 2k HPS just on yourself, ofc it doesn't have immediate effect of some other selfheal abilities or medpacs, but it's not supposed to do that.
http://parsely.io/parser/view/345004
Anni 2,5 mil parse, 1,8k HPS, 424k total healing over 4 minutes