A card for the min-maxer in all of us. Play this card in early scenarios to make your life more difficult, but reap the rewards of experience and upgraded cards in later scenarios.
Now the card seems deceptively simple, but there is a lot of nuance to playing it. Ideally you want to play it when you can afford extra actions, but not so early in the game that you aren't set up. Often this means holding on to your D2Ds until the last round of the game, triggering both and then ignoring the encounters you pulled (to the best of your ability) in order to finish the scenario. But there are other ways of mitigating this as well: a huge number of encounter cards are will tests and upgraded Mystics have access to Blood Pact which can be used as a freebee at the last doom in an agenda. Of course beware non-will tests and enemies that may pop out from the encounter deck instead.
The other nice thing about D2D is that it makes upgrading a little easier in the mid game. Because you get that experience as a permanent upgrade, in an 8-scenario campaign a D2D played in the first scenario is worth (1 exp 7 remaining games = 7 game-experience points) while one played in the penultimate scenario is worth (1 exp 1 game = 1 game-experience point) and a D2D played in the final scenario is just masochism (assuming you don't carry your experience through). For this reason you want to play your D2Ds early in a campaign, and swap them out when you have 3-4 scenarios remaining. This is especially important because they have no skill icons and as such are actually fully dead space in your deck unlike damn near every other card in the game (exceptions for Wendy and Ashcan Pete who can discard cards as part of their ability, of course).
So yes, get a little dangerous, plunge a few deeper mysteries and reap the benefits of playing on the wild side.