With that much RAM reserved, you'd have to assume some CPU cores are dedicated to OS tasks.
Possible reasons:
-Use skype for all video and voice chat on the new Xbox
-Facebook, twitter, email + other social media constantly running with notifications that can be checked
-Persistent Live Avatar world that no one cares about
-DVR features running in background
-High-res animated 3D blue-screen-of-death
-Fast switching between games and full dashboard apps (no more quitting your game to change console settings, check what got released on Xbox Video), or some form of multitasking in OS
-Full featured media player running in background
-Huge OS bloat that leads to poor performance and wasted resources
-Kinect processing (Audio, Video, skeletal junk) handled by OS
-Runs Windows 8 RT
-Full-featured Internet Explorer with tabs (with multitasking this could be nice for OCD gamers that use game guides)
Think Windows 8 tablet with fixed hardware reserved for running games. I'm guessing they're chasing more the "home entertainment" crowd, than just pure gaming. Just my 2 cents.