Contrary to what most Barbs say, it is not a fun skill. Yea it's fun in short bursts with the Insanity or Slaughter rune but ToC is a complete and utter chore. Just rush, rush, rush with total disregard for anything or anybody lest it run out.
However, there's a reason why it seems fun because it removes those tedious CC abilities.
Chain freezes, fears are much more not-fun than ToC is. Hence ToC wins that place on the skill bar.
Right now ToC feels mandatory for MP10. And for me personally it's very fatigue inducing to play like that.
I'd rather play MP7 without ToC than MP10 with ToC because it's more fun. But I'd be doing it knowing full well that my Legendary drop rates will suffer by as much as 75%. This shouldn't happen at all. One skill shouldn't feel so mandatory.
So go ahead Blizzard rework ToC, but rework those supremely annoying chain freezes and fears too.
Seriously, what kinda design decision was this? Give the hero abilities cooldowns but allow monsters to spam theirs?