An OD will not kick out of overdrive with a load on it. The kick down switch kills the engine momentarily to allow it to shift out of OD.
As long as you do not want it to kick down like a passing gear when you floor it then the switch is unnecessary.
All that is necessary to operate an OD completely satisfactorily by someone how understands mechanical things is the cable and a toggle switch. Then when you want to bring it out of OD all you do is turn the switch off and let up on the gas for a split second.
