Well imagine, i had two calendar applications on my computer just because i liked the features offered by the two, lets say outlook syned with my exchange and sunbird helped me with some other problem ( i cant imagine what sunbird will help me with.... but
lets assume it did).... imagine having how much will common store help you in this case!!! change made in one app will automatically reflect in the other.
Common data store can open up many possibilities for you as a user and certainly a lot for as a developer.
anyways, libraries doesnt really radically change the way we store data and relate the data in them and the ways apps interact with data.
i must admit that WinFS was really cool!!!
What's stopping Outlook and Sunbird from using a common format today?
Nothing. Except the same thing that would stop them from using a common schema in WinFS.