the primary reason to adopt a dual-currency system is to engage and monetize the two distinct sets of users: paying and non-paying. Paying users are willing to fork over a few bucks, cents, or in the case of completing ad offers, a few minutes of their time in order to get premium items within the game. These typically make up between 5 to 15% of your overall user base. The non-paying users, on the other hand, just want to play your game for free.
Inside Social Games features a guest post by people from Offerpal. The article outlines a number of scenarios and reasonings for different monetization solutions. Worth reading!