Keyboard and mouse support on console is a tricky thing to implement. Not in terms of drivers and functionality, the Xbox One runs a version of Windows 10 after all, but in the implication. A multiplayer round in a competitive shooter where one player is using a controller and another a keyboard/mouse combo could easily present an unfair balance. Which Microsoft is aware of.
The solution it seems will come down to choice and awareness. At PAX over the weekend Microsoft's Mike Ybarra answered a fan question to that effect.
"When we bring keyboard and mouse, we'll coach developers to say, look, you've got to think about your multiplayer pools. If you have a competitive game, people probably are going to want the choice to say, I'll play with other keyboard and mouse people, I'll play with only controller people, or you know what, I'll play with any of those."
Now, this sort of funcionality isn't a problem with certain games, playing Gears of War 4 cross-platform co-op with PC players isn't an issue at all. Competitive is the area where it becomes grey - so it's great to see Microsoft approaching this with the right attitude.