I don't really see the logic in Uber drivers discriminating too much about passenger ratings. If an Uber driver forgoes a 4.5* passenger, they're the ones indirectly paying through lost opportunity cost, expenses (in driving around driving around aimlessly) and their diminishing hourly earnings. Underutilisation is a massive cost to taxi/Uber drivers, so it's in their inventive to maximise it. Logic would suggest they should accept that passenger and the balance of probabilities which suggests the passenger will be probably be okay - I highly doubt a Uber driver could discern between a 4.5* and a 4.8* passenger anyway...
As Bolt said, you can't be rated 0, and on balance of probabilities equally, many default to 5.
Thus, reports from those who discriminate are that those below 4.5 are often due to apparent cause with things like lack of respect, entitled and/or unsafe demands and so on. Per the Uber announcement, they re-educate then kick off riders at the bottom end, so I don't think there are any riders below around a 4 (maybe high 3s).
I think many drivers prefer to avoid the risk of a passenger making a mess in their car, or the hassle of unsafe demands then being given a low rating (which has implications as a driver).
A number of those things could have more long term costs/effect than a short term skipping of an individual passenger.