2. Driver ratings replace the primary function of tipping, which is to incentivize good service and leave feedback.
That said, I very occasionally leave a tip in extraordinary circumstances, and would not be opposed to a way to do that in-app, as long as it was very clear it's not expected (hide behind 5-star rating, call it a "bonus" or something).
I feel terrible giving anyone a 4 or below because I know they might get banned from the platform and possibly lose their livelihood. Even if they're a mediocre driver, can't speak English, don't know where they're going and have a smelly car, I don't want that on my conscience that they get fired and can't feed their kids or pay for school.
Most people are aware of know how hard Uber is on their drivers and so now it's become that unless the driver commits an egregious act (road rage, wrong way down a 1 way street, unsafe practices, sexually inappropriate comments) everyone universally gets 5s. If Uber really wanted real rankings they would encourage ACTUAL feedback and have some sort of training and remediation process, not just banning people people if their rating drops below 4.7.
http://www.sarahdoody.com/wp-content/uploads/2014/03/lyft-ux...
I wonder how much that keeps it from being used.