This is an awesome idea if they can get it down pat. It really depends on how the public takes to the new method, and if they can get through it quickly. I imagine that the Cast Members have to constanly instruct the guests what to do, and ocassionally help them out if the system will not scan someone's pass. It may take some time to get the process standardized.... It will be interesting if they decide to use this technology.
And the idea of using them as fastpasses is a great one. Afterall they named them "The Key to the World". The Magic Your Way passes already can be used to charge things from the parks (a.k.a.- credit cards). It would be nice to be able to use them as a pre-loaded card of some sorts, though.