For features that are non-trivial to implement (>1 day), I encourage my engineers to push their WIP branches to our main repository so their work is not lost in the event of HDD crash/laptop theft/bear attack. Since we currently follow a mostly merge-based workflow, this usually isn't a problem.
Since Phabricator heavily encourages a rebase workflow (I'm always nervous making commits I plan to rebase publically available), and I'm actually liking it with the additional tool support Phabricator offers, it would be nice to have the concept of a 'work in progress' differential state for features that are not yet ready for formal review. I would imagine that the main difference in behavior is that it is not required to add reviewers to a WIP revision, and none would be notified until the revision was transitioned into a 'ready for review' state.
I would imagine that this would also be useful in obtaining feedback before a feature is ready to merge, and would collect such feedback in one place rather than having to chase down close audit tickets.