The only thing I can't figure out is why does Durango need 3GB for the OS? I fully understand that when the user is at the dashboard apps and etc. will need the full 8GB to expand and run properly. But, when I am playing Halo 5 I don't need to have Netflix and Internet Explorer running in the background. I only need party voice chat, messages, friends list, and the ability to stream music to be happy. Also, if the rumors are true that the next Kinect will have its own dedicated processor in the hardware then that should mean less drag on the Durango hardware itself. If the Wii U has 1GB of RAM dedicated to the OS and Orbis has 512MB to 1GB I just can't figure out where the other 2GB are coming from.
There are three scenarios I can possible think of:
1. This limit is set early on so that Microsoft has room to breathe for future developments. That leaked PDF from last year might be real and that special VR/AR stuff is a RAM hog or something.
2. Microsoft really likes multitasking. Durango can run the full OS at all times regardless if a console game is even being played. Possible marketing speak: "In less than two seconds the user can switch from playing a game to looking at the Netflix library."
3. This could be something of a soft limit. Meaning, that if you are developing some super powerful game without any Kinect (or whatever) support then you can ask Microsoft for access to an additional 1+GB of RAM. Ex. Call of Duty 12 needs the extra 1GB of RAM to allow for an additional 100 NPC enemies to attack you on the screen. But, the average XBLA developer doesn't exactly need that when developing the next puzzle game.
4. This limit is in place now but could be altered later in the console's life cycle. Microsoft could be playing it safe by mandating that 3GB will dedicated to the OS in 2013 but in 2016 when developers are constantly complaining of restrictions, and the OS ends up never takes up more than 2GB, then Microsoft might push out a major update allocating an additional 1GB for development. After all, it is always possible to give out an additional GB of RAM but not taking an additional GB of RAM back.