Putting it on iphone allows UberCab to collect a profile for all drivers and customers and make that profile value (ratings) publicly available for future riders and clients. Transparency is good but recorded transparency made publicaly available for future decisions becomes powerful.
This drastically idealizes UberCab profiles. That's easy to do now when UberCab is largely an abstraction, and the sole provider. It gets a lot shadier when UberCab is one of 10 companies doing this, when the market segments along lots of other attributes (high-end and low-end providers &c), and when it starts to become worth it to game profiles.
