Hmm, I wonder what could be different about the devs' machines that makes this feature work, but on our machines it simply doesn't work at all. This is a brand new installation, and my settings aren't unreasonable at all. It's really strange. They wouldn't have released this beta and announced this feature if it didn't appear to them to be working, yet for at least some of us it simply fails to work, and doesn't announce any errors or anything. It's just not there.
I actually installed the 2.4 beta specifically for this feature. I use Cura most of the time for most of my prints, but I also use Slic3r, Craftware, or Simplify3D if they can do something I want with a specific print better than the others can. I'd been printing this frame most recently with Simplify3D, but it's process model is broken for what I want (has to do with top and bottom layers and how they (don't) work with S3D processes).
Cura's gradual infill option is really cool for what I'm doing, and this cubic subdivision would have taken what I gain from gradual infill and extended it in all directions inside the model, which is just great.
The model shown is a quadcopter frame. On a quadcopter frame one must balance weight and structural rigidity and strength. The cubic subdivision would allow me to move more or the infill structure out closer to the walls, where it would increase rigidity (and help top or overhanging layers go down better), while hollowing out the center, which reduces weight. This is literally the only feature that made me go through the crap of uninstalling the previous version of Cura (2.3.0) and removing the data directory*. If I can't get this feature working properly I'll continue printing this model using Simplify3D, which can do a couple of things that I actually need better than Cura**.
*this is what I had to do to get Cura 2.4 to launch at all, since Cura still, at this date in 2016, hasn't figured out how to get along with other versions of itself on a user's machine
**one of those is specifying two-layer brim for adhesion. The Taulman Alloy 910 nylon I'm using to print these frames likes to lift up a little around the edges of the ends of the arms if only single-layer brim is specified, but a two-layer brim locks it down pretty well