Phil Spencer: cross-buy will become a standard feature of Xbox
To clarify this, the cross-buy is the feature whereby, bought a game in digital, you have it available on all devices on which this game is present, as long as you log in with the same account used for the purchase. In this case we speak then of games on both the Xbox and Windows store 10: purchasing them on one of the two store with our Live account, they will automatically be downloaded from the other store.
Spencer has, however, made it clear that this is not necessarily a feature limited to Microsoft games, but will also benefit third parties: see a CoD on Xbox would then also have the PC version and so on. All this would still be subject to the approval of the editors, who may decide whether or not to take advantage of that opportunity; for example the cross-buy is very present on the PlayStation Store for games that come out on both PS Vita that PS3 and PS4, but not all benefit from it and it often happens that if you want to use the same title across multiple platforms, all bought multiple times.
We just have to wait until then to find out when this feature will be released, and what publishers/developers decide to take advantage of it.