Being an engineer, I typically try to figure out a lot of things....I'm not an auto design engineer....
To me, a shackle is just an extension of the main leaf spring. It's design is to provide displacement of the spring during suspension cycling. The frame location and length are very important. The shackle is to provide a swing motion to when the spring transfers from a concave configuration to a flat configuration to a convex configuration (during movement. Only problem with just adding a longer shackle is that you won't get the benefit of the full extension of the flat leaf and you also have potential at full droop not to have the shackle eyes centerline in line with the axle mount. If the centerline of the eyes go past the axle mount, then the leaf will bind and snap against the frame in front of the shackle mount. This obviously damages the leaf and places lots of shear stress on the shackle mount.
Note this, the different length leafs provide the different load carrying capacity of the pack. The more the droop the less capacity, then at some point, no capacity, and lastly, if drooped far enough, you have resistance.
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