If pulls a 5-star service for 499 customers and a 0-star service for a single customer (for whatever reason) his rating drops from 5 to 4.99, which doesn't seem that relevant. Unless a 0.01 drop in his rating means a loss in earnings greater than that of a lost tip, the tipping system would provide better rewards for not dropping the ball.
Forgot about the social pressure, too (I'm not American). That could be a real issue, because if people don't actually consider not tipping then it's just dead as a incentive scheme. They'd have to make tipping anonymous.
By the way, I don't think pressure under anonymous choice is "social". You could attribute it to education and thus society and some way but ... that's rather roundabout. I think the more appropriate term is "consciousness".
Granted, the rating system is no better, even if for different reasons: since Uber/Lyft will deactivate drivers if their rating falls below what I'd consider to be a fairly high value (something above 4), most people (where I live, at least) give 5 stars unless there was something seriously wrong with the ride, so a mediocre driver will get the same rating as an exemplary driver.
Doesn't the rating system approximate this outcome well enough, though? Being able to rate a driver, and see the overall rating before I get in the car is of huge value to me as a customer. Feeling obligated to tip is a net negative for me.
In a restaurant (in the US at least) it's so expected to tip that if you tip lower than 15% for terrible service you're considered cheapskate. Even though Uber's tipping will presumably be anonymous-ish (assuming they implement it like Lyft has), I would still feel that social pressure.