The answer to why is there a need for a 70C discharge rate for a battery is helicopters. Not control line, not even fixed wing R/C, but the batteries we use 20 to 40 C will explode in a heli application. When they hit the throttle and the collective engages the load on the motor is serious, resulting in a surge of current that can only be handled by very high C rated batteries. I have yet to see an application for a 70C battery in control line, and as was mentioned before to get that C rating requires very heavy cells.
Andy