The MPD from CMC is designed to be used as a belay device to arrest a falling load should the main line fail. A very common question we field is regarding the “shared tension systems”. Shared tension systems are essentially two lowering systems in place of one lowering system and the belay. If either system should fail, the increased load on the other descent control device will cause an increase in rate of descent. The MPD is an effective belay device capable of safely arresting the load.
During the design process the MPD was given one amazing high efficiency pulley with a One-Way Bearing system to it. Due to the design of the MPD, when using the it as a progress capture and primary pulley during the building of a mechanical advantage system, the efficiency side of the equation is very fluid.
You save a bunch of money and no small amount of weight when you figure the equivalent equipment needed to do what the MPD can do for you and what it gives you. It is faster, simpler, and quicker. The MPD has few components, is less weight and has lower risks involved.
Some of this information was taken from CMC Rope Rescue Field Guide 4th Edition
Peace on your days…