I've run Promise of Power in a couple campaigns now and I've been really impressed. I don't think you need to have any Curse payoffs in your deck to happily play it. It's just a good deal in its own right.
I think Promise of Power is considerably better than Unexpected Courage, especially on higher difficulty levels. Getting +4 to any test for any investigator at your location is premium--compare cards like Rise to the Occasion and Inquiring Mind, which (in my opinion) feature much larger downsides, while only giving +3! With a +4 bonus, even when playing on Expert, you have a real chance at passing a skill test in a weak stat. That's a nice impact for one card, and something like Unexpected Courage is simply not powerful enough to achieve this.
The Wild icons provide a welcome element of flexibility to Mystics who are otherwise focused only on Willpower. Scenarios often feature important non-Willpower tests, which can be awkward for hyper-specialized Mystics. This card is a great answer to that, and of course also has the flexibility to help allies cover for their own weaknesses.
Moreover, a single curse token in the deck is a very small cost. There's a very high chance that it gets discarded on a test that passes anyway (or, less probably, on a test that would have failed anyway.) I will happily take a large bonus to my skill test right now, when I know the test is important and I know the bonus is useful, in exchange for a smaller penalty later, which might come up on an unimportant test, on an important test that I committed a lot to and therefore can succeed on anyway, or simply not come up at all.
Finally, this is Practiced. That's very relevant because Practice Makes Perfect is a great card, significantly above the curve for a level 0 card and well worth building around. Together with Deduction and Perception, this makes a really nice package for investigators like Amanda Sharpe and Daisy Walker.