Scubyfan-

The reason for using on-board glow is to keep the plugs "hot" during low RPM/idle, which should help engine reliability. With the controller you can program it so that the glow is energized only during idle conditions, which saves battery time and is in theory easier on the glow plugs.

I don't have onboard glow on my Twin-Air because my OS 46's are plenty reliable at idle and the plane is supposed to be relatively easy to handle on one motor. But I did install onboard glow on my Mustang's twin-cylinder Saito to prevent a cylinder from dropping out at low RPM.

By the way, the standard electrodynamics twin setup uses a 3V battery wired in series through the glow plugs as Bill describes. The controller is designed to handle this.