[peacetreefrog]

Producer surplus by definition is benefit to the drivers. It's the amount actually make minus the minimum amount they'd be willing to take. This paper purports to show how the minimum drivers are willing to accept (their reservation price) changes by the hour. Flexibility -- fact they can choose to work when their reservation price is the lowest, or only lower than what they'd actually make -- is definitely a benefit.

Obviously a driver's alternatives are a big factor in whether/how much to drive, but flexibility might even MORE valuable to drivers who have other jobs, because they can work around it.