Good question. I still wanna be able to play against mates on COD & GTAIV etc when they have long upgraded and I'm still lagging behind on my old original 360!
It's really a good question. When you say that, I think COD, but they are lazy at Activision.
It would be really nice, but every time, developers do nothing. Especially at the beginning of the Xbox One, it would be epic. I still think the Xbox One work differently on Xbox Live, so there is little chance to witness this kind of functionality.
I have not decided if I will stay with Xbox or try PS when I decide to upgrade, before the ability to play my old games on my system would keep me with Microsoft. Now that I need to buy all new games switching is easier if I want to. If I can't play with my friends that have the old Xbox system once I buy the ONE, I might think more about the PS4.