Saw these in a catalog I got from Lee Valley recently and thought they were a cool idea and might be something good to have.
I'm pretty religious about tieing down my load when I go to the wood store or carry anything that has a chance of falling out of the bed of the truck. Generally I will make a sheepshank knot like this . . .
in the rope, run the loose end through a hold down on the truck and back up to the knot, through it and use it as a block and tackle to get the whole setup tighter before tieing off the end. Done right, it is a quick operation to unharness the load at the other end of the trip and you just shake the sheepshank out of the rope, and stow the rope.
That figure 9 rope tensioner would take the place of the sheepshank and be much quicker.
Had to order some specialty hardware from LV today, so went ahead and added two of em to my order.