
There are a couple of points I don't see mentioned but I think are fairly important to consider when thinking of taking either this or it's lvl 0 counterpart.
The first is that the card DOES NOT protect you from negative effects that would occur even if you succeed. The big ones being the card loss from Mists of R'lyeh, and the action loss from Rite of Seeking. While the FAQ does mention Baseball Bat, that's a card, so I thought it worth mentioning that you will still have to deal with the consequence of a fickle spell cast. If you really can't afford to give up a card or you really need to investigate as your first action with RoS, Defiance will not help you. You will also be forced to spend a charge from Seal of the Seventh Sign.
To explain, Defiance blocks the effects a symbol token attempts to do as indicated by the scenario reference card; it is otherwise ambivalent towards your assets and your cards in general. Which brings me to Ritual Candles.
If you are using Ritual Candles, Defiance WILL NOT impede on the +1 skill boost. In some (albeit unlikely) cases, you may use this in tandem with two candles and then trigger, say Olive McBride or Dark Prophecy, to then turn what would have been a -4 (or , , ) into a +2. That's a significant difference. Part of the reason Olive is useful for this is that she can absorb the hit from the if necessary. And if you have access to say, cards, then you can confidently trigger succeed by 2 effects and other combos with greater assurance. And this does differ from Counterspell which, while protecting you from spell side effects, costs two resources and will definitely prevent candle bonuses. Though, I'm getting ahead of myself. Probably you'll just want to remember that Defiance doesn't disrupt your Jewel of Aureolus.
Lvl 0 Defiance can be used to target a popular token in this way, or to debuff a particularly dangerous token, but the choice may not be so easy, especially on harder difficulties. There is the possibly of it serving the measly bonus of being just a since no such symbol token may be drawn. This upgrade provides the opportunity to play cards that draw lots of tokens or hunt for specific symbol tokens in confidence because the is certainly not going to waste. Just remember that all consequences to symbol tokens from your play area will remain in effect.