If you add it to, say, strength or skill ed, it would almost disappear, and having to stay at max fury to gain that benefit restricts your build quite a bit. I think most abilities like this are multiplicative (maybe not with each other; we'll see about that). IE, weapon damage*skill ED*strength ED*berserker rage.
Well my way of thinking is that it gives more significant boost to those less significant skills.
So making for example basic bash damage increase from 150% to 175%, or frenzy 110% up to 135% or Rend 210% up to 235%.
It would be then multiplied by straight and all and make dome difference, don't you think?
Only earthquake? What about Hammer being not 250% but 500% with WotB with insanity? What about ancients doing 400% Dmg with their specials? About Revenge doing 440%? Think that if Wotb Insanity is calculated this way, its IMBA.