There's really no reason for the passengers to know the license plate number of the car that's coming until it actually arrives. Most people don't even look at the plate anyway. Upon arrival, it should only give out the 3 digits of the plate because that should be enough to positively identify the car. Someone could build a list of license plate numbers of cars doing Uber and use it against them for insurance purposes. Also, someone could run the plates on a number of websites and find out where a driver lives.