Torrent of Power single handedly justifies the inclusion of spells that HAD been very fringe prior to its release.
You see, before this card, any spell outside of Shrivelling and Rite of Seeking was considered "filler". I am thinking of Scrying, Rite of Seeking and Clarity of Mind.
This card lets you turn those slots that had been inefficient into a bank for these cards.
This card is particularly good because it is modular. You CAN spend an extra charge off of your key spells (Shrivelling and ROS) to get a slightly more powerful Unexpected Courage, but it really doesn't do this card justice.
No, to get the most out of it, you will have to play some other spell and let it sit out as a bank for this card so you have the potential to just blow a key test out completely.
This card is also fantastic on higher difficulties as you can reliably spend up to what you need for those key tests.
I would consider this card in most Mystics, but it will require that you include a third spell into your builds aside from Rite of Seeking and Shrivelling in order to really get the most out of it.