I think there are many considerations to think about, depending on why somebody wants a different hitch.
I'm in the process of fabricating a hitch right now, actually. I want to use implements that don't fit with the Cub's mounting system (such as various rear cultivating tools, tine weeder, bed preparer, etc), and I want to be able to adjust things easily. And I want to be able to change implements quickly with pins, not bolts or clamps, since there can be a number of implement switches over the course of a day of farming.
Using pins means there's no longer a solid connection between the implement and hitch (like a fast hitch point, or clamps, or the Cub 6's toolbar welded to the arms), so the implement spins on the two lower link pins, requiring the third point (toplink) to keep the implement level. Also the toplink can adjust the pitch of the implement, and since the hitch can be a parallel-ish linkage, the angle of the implement relative to the ground doesn't change much when it's raised or lowered. A solid connection between implement and hitch means the implement raises in an arc, which is less useful. I'm designing mine with lower link arms that extend under the tractor (similar to the Cub 6's), so that the hydraulic system gets better leverage when lifting something. That makes it less of a parallel linkage, but not too far off where it counts.
A fast hitch has many of these functions, but I can't make one, and you can't straddle a row of crops with a fast hitch.
I didn't start out to create a modern 3-pt hitch, just a hitch happens to have 3 points, because that makes it useful to me. It may or may not be able to accept modern 3-pt implements, because their spacing isn't what fits the Cub, and my lower link arms are prevented from swinging side-to-side, which makes it hard to move the arm over to slip the ball over the pin! I'm using a different pin setup.
I'm also fabricating some belly-mounted implements which will also use a three-point mounting setup, for the same reasons.
I think there are few modern implements that would suit a Cub, even with a modern 3-pt hitch, but that there are lots of reasons to figure out a better hitch system for the Cub.
Barnyard, I appreciate your pictures! I hadn't thought to build a new rockshaft, too, but doing it like the fast hitch like you did makes a lot of sense.