It can pay back it's cost in 2 rounds pretty easily if you have 4 creatures, which is faster than any other economy card in the game besides Meditation Amulet. In that sense, it's actually a better mid game economy card than the other possibilities.
I don't use it anymore because it's an additional investment in the Barracks, and the Barracks already forces you to commit heavily to it before it operates efficiently. Once the Barracks is gone, you're reduced to spending that mana on the Battle Forge or Gurmash. I haven't found Gurmash to be particularly exciting, and flooding him with mana won't change that in my opinion. Battle Forge benefits somewhat from the mana, but by the time you've played a Barracks, 2 other outposts, 4 creatures, Battle Forge, and Altar of Carnage, you should have enough equipment played that most of the time the extra mana will be of somewhat limited use.
Besides, if my opponent leaves me and my conjurations alone to build when I'm generating 17 mana/round, I'll win anyway, no altar needed.