Long story short: the container terminal will always opt to please their customer (shipping line) over their non-customer (trucking companies). Truck appointment systems are usually used to force transporters to smooth out peak times not in the name of efficiency, but rather to lower the amount of dock workers the container terminal needs to hire. The truck companies generally end up footing the bill for this, both in increased workload and in detention/demurrage costs because they can't get their containers out and back in time. This money goes directly into the pocket of both the shipping line and container terminals as this is typically something they make heavy profits on.
Be very wary when container terminals and shipping lines start to push for centrally mandated appointment systems. They are much more consolidated than hinterland transport operators. I'm all for increasing efficiency but let's not even further increase market power for shipping lines and container terminals please.