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Returnless fuel systems are mainly there to prevent evaporative fuel emissions. Evaporated unburnt fuel is a component of air pollution.
There are two different styles of returnless fuel systems being made right now.
One, produced by Ford and Kia (and perhaps others, I don't know) uses a fuel pressure sensor and varying the drive duty cycle to the electric fuel pump to maintain a fuel pressure that is indexed to manifold vacuum. The fuel injectors would see a constant differential fuel pressure across them, regardless of intake vacuum. This fuel system would use the PID (proportional-integral-differential) control system algorithm that OP is mentioning.
The other is produced by Chrysler. It uses a traditional relay to control power to the fuel pump, and a fuel pressure regulator that has no vacuum port. That fuel system maintains a pressure that is a constant gauge pressure of 58 psig (49.5 psig on older models). Of course, this results in variable differential fuel pressure across the fuel injectors. Chrysler engine computers take this differential pressure into account when calculating injector open times. Thus, it is not necessary to develop any sort of control algorithm for the fuel pump.
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