Look again at the centerfold. You can see that all the parts tie into the D-ring. Because of this configuration, each part is short or small. When time comes to replace something, you have a smaller repair to make. You would not have to replace an entire trace, but only the portion that needed replacement.
Repairs are easier and less expensive.
The weight of a heavy tongued implement like a mower is not carried only by the horse's neck. This point really sets the D-ring harness apart from all the rest, as it eases the tongue weight load from the horse's neck. Only because of the D-ring can it do this. Properly adjusted, the front sidebacker straps are held under tension by the back trace. The front sidebacker straps are thus pulled tight between the neck yoke and the evener, transferring a goodly amount of tongue weight to the back pad through the D-ring.