It is bundled, but you can choose to not plug it [1]. Kinect is useful in games like fitness and rival sports, besides obviously being used to add other functionality. Obviously, privacy implications are huge.

[1] - http://www.theverge.com/2013/8/12/4615586/xbox-one-no-longer...

It is bundled, not required as of August. You can disable the Kinect in the settings or unplug it completely and the console will still function: http://www.forbes.com/sites/erikkain/2013/08/12/microsoft-re...