Well, how much power it loses for games to the "platform" computing resources depends on what kind of hardware it carries. The platform resources might be mobile style CPU/GPU. What you gain is responsiveness. If you have the application running on segmented resources, and they can run exclusively, devs no longer have to worry about all of those other services running on the resources they're trying to use. That hopefully means no delay when you press the guide button to check check which friends you have online, join a party, send or receive voice/text/video messages, check the status of downloads etc. That could also be a big win for notifications, if there is a unified notification center for any social media apps the platform might support, or other apps that push notifications. You could have a sports scores app that pops up updates to a game you're tracking, where you could instantly open the guide and check out the full details and then switch back to your game, all with little to no pausing or delay. There would be no lost resources to high-quality group voice or video chat. Maybe I could have a 1-to-1 video or Skype chat, picture-in-picture, while playing a game without any impact to performance. Maybe even picture-in-picture television. I don't think I'd want to do that, but someone might. Kinect processing, or some of it, may count as a "platform" service, which would mean any game that utilizes Kinect would be making use of those "platform" resources. We'll see what the memory split ends up being, if this is actually the design of the new Xbox, but I have a feeling the platform CPU/GPU something along the lines of a phone/tablet, rather than a significant chunk of the overall computing power of the system. Overall, it's an interesting design concept for quality of service guarantees while running intensive multimedia applications like games. It's kind of a hard multi-tasking system, rather than a soft system like you'd have on a PC OS. The "platform" side could run with some kind of scheduler for multitasking within its own resources, but those "application" resources are always fully dedicated (non-multitasking) to whatever game you're playing. Hell, this console could run a virus scanner on the platform side without impacting game performance. How 'bout that?
I would like to point out, this is all entirely speculation and not "insider" info of any kind. I'm just hypothesizing about why this might be the direction they've chosen for the new Xbox.