I wonder if this is the first step towards offering a "limit" option. For example, where I live the average price from our house (Jersey City) to the mall is $7, but some days it jumps up to $15 or more. My wife and I usually choose to walk those days, but it would be nice to say "hey we want to go to the mall today, let's set a $7 limit and when that hits we'll go." For the flexible consumer it's easy to wait for a better price and not need to constantly look at the phone to see what the current market price is. The limit price could help create a queue which could provide a driver with a continuous supply of rides during non-peak hours.