Is Uber's star rating system an effective tool for weeding out bad drivers?

It definitely works.  It weeds out those who don't care.  The ratings isn't for experienced drivers who may have had prior experience with taxis and limos.  They will do fine. They know how to handle customers and provde the quality service.

Uber signs up quite a number of drivers, and they really don't check for experience being in the service industry with customers.   This means there are regular folks who has never done this before.  Uber needs a way to make people care about customer service and not let them treat it like it's just a money making platform for transporting goods.

It encourages a higher standard of customer service.  Newbies will need to work harder at the level of experienced drivers. and I think taht's a good thing.

One can argue whether it does anything besides letting Uber know of the real bad ones which enable them to deactivate the bad apples.  I mean, it's not like the Amazon marketplace where customers have a choice between which product to buy.

I have heard that if you have good ratings you get matched up with good riders with good ratings.  However, I am not sure if there's that much truth to that from my experience.

I think it works pretty well.  With everything considered, I think Uber does a decent job at maintaining this rating system.  

Sure there are some a-hole riders who give real bad ratings, but Uber allows those to be disputed, and eventually with enough rides, good drivers will have good rankings and bad ones won't.