The only thing that could be an advantage some would see is 10% extra mitigation from the 10 seconds of Dark Mind. But it's really not an advantage at all. War has overall, all around better constant mitigation. IB a few times in between busters. Vengeance + IB + Equilibrium every other buster. Thrill+Conv (<-AKA War's Rampart) + IB + Equilibrium every other other buster, and rotate between those with IB's in between, and for every buster, as well as equilibrium.
Drk can only rely on Dark Mind every buster. If they string any CD's together like a War can, they wont have cd's to string together for the following one. You'll be left without something to use for a buster now and then, because your only reliable CD's are Shadowskin and Shadow Wall. There will always be a point where a tank buster will come in, and both of those will be down.
It's literally just a trade off of taking 10% less damage on 1 buster, to take more damage on the next one. And this is if you actually save your CD's specifically for tank busters. A War has the leisure of keeping mitigation going even inbetween the tank busters.
And lets not forget, Discord or perpetual ray or whatever the actual buster is called, hits I believe it's 3 or 4 times. Meaning a single Equilibrium between those hits, especially one buffed by Berserk, would completely nullify one of those hits all together, probably even more. And this is on every buster.
