You can get more efficient forms of the spells. For example, I am death. (just making up numbers) I can spend 1 pip to kill my minion and get 300 health. or i can get the ice spell and sacrifice the same minion for 400 health for one pip. The same applies with the power spells.
I personally HATE those spells SO much however I do have take power in my book but not in my deck I just want the master of fire badge Oh and btw steal health doesn't sacrifice your minion, it takes away 300 health away from the minion and gives it to you. So it only sacrifices 3 out of the 14.