The Xbox One already supports USB keyboards for text input, but it isn't generally used for gaming on the console due to the lack of mouse support for camera control.
For PC to Xbox streaming to work correctly, adding mouse and keyboard support is a given. However, I think it's interesting to note that Phil said mouse support isn't "far away" after saying there's no plan in place for PC to Xbox streaming. If mouse support is on the roadmap before PC to Xbox One streaming, it could be intended specifically for native games.
Even if that's correct, I think it's safe to say that mouse and keyboard players won't come face to face with players using joystick controllers in competitive games. The advantages of a high-DPI mouse far outstrips the Xbox One's precision joysticks in FPS, for example. Besides streaming, mouse support could open up the console to ports of more complex games typical of PC, such as keybind-heavy MMOs or strategy games. Could we finally see World of Warcraft on console? A true keyboard and mouse RTS sequel to Halo Wars? Only time will tell.
Phil Spencer also mentioned that Windows 10 for Xbox One is "on schedule", and that we'd be hearing more about time scales soon. It stands to reason that we could hear more about Microsoft's Windows 10 plans for Xbox One at their upcoming Gamescom 2015 briefing on August 4th.