If you wanted to look at this from the perspective of MS, my suggestion would be to consider it starting with the transaction fees. You will have to pay some percentage in order to convert your bitcoins back to fiat. There is of course a lot of risk in the value of the Bitcoins (they could go up, or they could go down) - but even if they don't move, is this strategy better than existing opportunities where you can swipe your card, eat a transaction fee, and take home points?