Motors in series do not always share the load evenly even if they are identical. To witness this yourself, turn an engine upside down and run it. If the motors are in parallel,stopping one truck with your thumb will not affect the other truck speed very much. If the motors are in series,stalling one truck will result in the other truck speeding up. On a sharp curve approach this could happen during operation. Can pull of traction tires with one truck dragging the other.
A PWZW puts out 20 volts. 2 motors in series would be limited to 10 volts. Cruise control locos would allow even less. 2 motors in parallel would get 20 volts minus the diode drop. If 4 diodes are used in series that is about 17.5 volts without cruise. Most people in conventional don't run the motors even that high so there is no effective loss of pulling power. In effect you are limiting the top throttle just a bit. If cruise equipped with voltage held in reserve it might be a slight issue,but much less so than series wiring IMO.
Dale H
