The traditional two battery and switch setup has one safety drawback (OK, glow engines too) If power to the receiver is lost due to a bad connection in the battery or switch harness. No matter what the fail safe setting the receiver provides, the engine will continue at whatever power setting in affect at the time the power to the receiver was lost.
In an IBEC setup, if power to the receiver is lost for any reason, the engine quits immediately instead of continuing at whatever the throttle setting was at the moment the power was lost.
Pete