You are right and wrong about avoidance. Yes avoidance is unpredictable. But keep in mind, any tank can be 3 hit dead on Lordearon, even through 20% damage decrease cooldowns : Sindragosa hits for 35k. 3x hits is 105k, bears have around 70k HP while paladins are around 56k. So what do you aim at?
- Reliabable EHP from heal increase? We saw that 4-5% heal increase is in the range of 1.6k - 5k worth of EHP, depending on scenario. You'll still die.
- Unreliable Avoidance by reducing the worst case chance (the 3hits in a row) by a relative 8%? You'll unreliably survive.
The EHP solution may or may not save you. Usually, its on the "may not save you" side. 3 hits in a row mean 105k damage, with 56k HP you'll need 49k healing to happen to save you from dying, and another 40k to get you back to a points where you are able to survive the next blow, all this in the 3.6s+1.8s window of the 3+1x hits. That's 7 non-crit Holy Lights or non-crit Healing Waves (assuming ES/PoM/Seed/hots will have ticked for 10k, reliably).
The avoidance solution may not save you either, but it reduce the chance of that worst scenario by "a lot" and therefore ensures higher uptime on the Ardent Defender talent "life saving" part and therefore... makes you live a little bit more often.