it's not necessery...
if you're doing 1k dam with it/90sec (that's what i get fully buffed crit) you're getting 11dps from it....but you can only use it when target hp is lower than 20%, and you're not gonna crit it every time, for max damage, so the dps is significantly lower.
So on a normal dungeon boss, you might get 3 dps from it, and because of RNG, it probably wont make any difference.
but if you have it, there's no reason not to use it; if you don't have it, there's not much point in getting it.