To make a better cube.any way why would you want to hybrid it?
Type D core doesn't cut corners, and you can't get the screws all the way in. This results in a cube that jams a little bit and is pretty loose (for 100% Type D cube).
The old Type A core is good at cutting corners, and will let you get the screws all the way in. This results in a cube that barely jams and almost never pops (for Type D cube with old Type A core).