Without the idler cable, the rear chain/cable becomes the primary driver. It pulls the rear sprocket/fin across AND operates the opposite drum which in turn pulls the front fin across in the opposite direction. This not only adds strain to the rear chain/cable but also tends to jerk the front chain back and forth due to normal slack in the system.
The idler cable transfers part of the load from the active drum directly to the opposite drum, distributing the stress and making the operation of the Drive smoother. As you know, the Drive can operate without it but tends to feel a little jerkier and is harder on the chains and remaining cables.
In addition, you'll get better reliability and greater longevity from your Drive by using all three cables and making sure they are properly adjusted.