Drivers will still take advantage of such a system. Whenever a market is over-supplied, drivers will sign in to collect their free "on-call duty" checks for doing nothing.
There are all kinds of adjustments you can make.
Another thing: you don't get paid for "doing nothing", you are getting paid for dropping everything when needed. On these conditions you're limited in all kinds of ways, e.g.
Taking care of a small kid alone? - not possible. Going shopping? - not really possible.