Rappelling with this set up and a laid rope is a very bumpy experience. I was surprised to feel a constant vibration through the system when I tested out this cool rack. The chain links between the two carabiners provide a massive amount of friction giving the climber a lot of control during a descent but creates a flat area on which the twists in an old rope bounce around.
When using a single laid rope adequate friction is still created but, as is common with these old ropes, twisting and spinning is a problem. In a free hanging rappel a climber would get a 360 degree view of their surroundings. In double rope applications the spinning and twisting issue is not so pronounced but there is so much friction in the system the climber sometimes has to feed the rope through in order to move.
Using three chain links to connect the carabiners is not simply for added friction, strength, or security. Extra links enable a climber to leave them at anchor points to serve as rappel rings.