"Dark Knight: Tanking On The Edge"

[Alphras]

If you can find math to the contrary I will edit it out. However as I understand it Shield Oath and Grit are additive to balance with Defiance. If they are multiplicative with your cooldowns it would -seem- to mean that a 20% mitigation CD like Inner Beast or Rampart would be more eHP mitigation on WAR than it would on PLD or DRK. Math is not my strong suit though, I fully admit.

It doesn't. A multiplicative 20% damage reduction is exactly the same as a 25% hp increase (in terms of eHP).

1/0.8 = 1*1.25


To put it into an example:

If you take a DRK and a WAR with a base hp of 15k.

The DRK activates Grit and Shadow Wall, which reduces incoming damage by 44% total.
The total damage he can take (his eHP) is 15,000 / 0,8 / 0,7 ≈ 26,756

The WAR activates Defiance (which increases his HP to 18.75k) and Vengeance (30% DR).
eHP = 18,750 / 0,7 ≈ 26,756

Its just that there are SO MANY benefits to having it off, particularly when MTing, and nothing particularly special about having it on.

While I agree somewhat with your assessment of Grit, there is Souleater that requires Grit for the healing effect. So there is at least some extra incentive to turn it on (beside the 20% DR and the enmity multiplier).