How would this work for apps that are multi-user, like email, social networking, or marketplaces? The Sandstorm data model doesn't really work when you're dealing with the data of many users besides yourself.
There have been attempts to build federated social networks over the past few years, such as Diaspora, but they have not really taken off primarily because it's too hard to get users to run their own servers. Sandstorm potentially fixes that.