The added functionality was from a request from a rigger friend of mine. As a rigger, he and most other riggers he works with find that usually when you display axis, you expect them to be at the root of the bone (like you'd expect the axis to be at the origin of an object) vs the tip. However, there was a concern that having the axis at the root of the bone would get hidden in areas where there are plenty of other bones in the same area. So, the proposed fix is to have the axis be able to be moved from the root to tip of the bone and leave the positioning up to the rigger.
Right now, the axis position is in the same UI as the show axis property. From my understanding, it sounds like the UI team wants to keep those two elements close together. Also, the slider is a factor, since that seems to be the design that most people agree is better. Lastly, the UI is borrowing from the camera passepartout, and generally I'm trying to follow Blender UI conventions as closely as I can.
Screenshot:





