Glad you asked! And Jessica Mulligan gave a reply:
2. It enables us to port back the new user interface code from the Core software. This is the UI code being used in MEO and it is pretty nice. It will give us the ability to more quickly add capabilities to the UI. Right now, anytime anyone mentions messing with the UI, people tend to turn white and start looking for a place to hide.
Now go read the other advantages of going to DirectX9 on the Turbine boards.