The Xbox One OS developers know that this is a must have missing feature and according to recent accounts they are working on resolving the issue for an upcoming update. Apparently they must be getting close as Xbox head-honcho Phil Spencer tweeted out that he has been playing with the screenshot feature and it must be close to getting released.
While Spencer gave no indication of a timeline or when we might see it, the fact that they have it working on the Xbox Dev Kit pretty well has to be a good sign. We have to believe that it will be in one of the updates just ahead in the near future.