A brilliant idea but it will not be that easy to implement and spread it !! first factor being developers won't devote their time in developing apps that can only be purchased by botcoins rather they will upload it on play store to get more customers and exposure and even if they upload their app in both the markets i think people will prefer getting it from play store so the first factor will be getting apps which are new and only available in the bitcoin app market and finding devoted developers for that also to check and test every submission weather its clean or not and a huge press release will be much needed according to me !!!