I'm mearly pointing out the differences, if anything the DB's would be inferior in regards to full resistance at contraction position as I stated earlier.
With DB's you can automically vary the degree of resistance as you stated by adjusting the moment arm.
With a cable, you cannot, the resistance is not rotational but in a straight line away from the torso..
The limiting factor of a "pec-dec" although rotational and with the assistance of a cam to automatically vary the force thruogh the ROM will not allow the arm to cross over the mid-line of the torso because of the pads, a really well designed machine would allow the arms to move iso-laterally, but thats rare in modern machines.
At the end of the day, DB's for practicality and efficiancy.