At the moment Phrequent isn't really powerful in terms of reporting. Integration with Harvest means we don't have to think about those kinds of issues right now, and can delegate all the complex stuff out to a third-party that generally has more experience in this area.
It has the added advantage that Phabricator tasks can be tracked as time in Harvest, which would be useful at least in my scenario (where the official time tracking occurs in Harvest).
We'd need to support "Login with Harvest" in order to grab the appropriate OAuth details for performing API requests on behalf of the user. I think it's safe to assume at this point there's a pretty general OAuth implementation that everything derives from; @epriestley can probably confirm this.
[[ https://secure.phabricator.com/chatlog/channel/6/?at=85200 | Reference chatlog ]] for details on Doorkeeper structure and integration requirements.