Just re-read your post here and I think you may be confused as to what the 'Ford" solenoid does - it's generally used for the large B+ wire that goes to the starter. It "only allows the B+ wire to be hot when cranking", or so the saying goes. Once the car is done cranking and is running, the solenoid goes dead, the only hot wire left is the B+ wire to the lug on the solenoid. And if you have the switch wired correctly, that wire will be dead as well when the switch is thrown.
I used it because I liked that idea and wanted to run the wire inside my car. If it's a race-only car, you could probably get away with it outside tucked up under the frame rail to the starter but mine's a street car so I wanted it inside. The caveat to having it inside and live all the time is that it's a potential hazard - it's a big wire with a lot of current flowing through it. If it gets shorted etc., it could be a big problem with fire.
The CD solenoid really has nothing to do with the Ford solenoid, they handle two separate issues. You can have one without the other, both or neither depending on how you have your cut off switch set up. I just happen to have both mounted in close proximity because they need to be close to the switch and battery to do their thing.