They are the best way to actually visualize the needs and wants of users. They are also fun to write. They have the drawbacks of being (a) fictional (b) focused on extremes (c) not really provably statistically valid. For these reasons we also need statistics to tell us more about the effects we want to have, the victims and users whose choices affect the victims.
See Simple English User Stories for examples of how this technique is being used in other open wikis.