If Kinect 2 was optional:
It would be $100-$200 cheaper to buy.
People who want Kinect 2 can always get one.
No worries over privacy issues.
The are NO launch titles which have features which seem to need Kinect 2 (nobody has come forward with anything yet you couldn't just as easily do on a normal controller).
Indeed I have not seen ONE Kinect 2 exclusive title yet.
So why not just make a new SKU?:
Xbox One (Core): $375 - Console, Controller, Headset
Xbox One (Premium: $500 - Console, KInect 2, Controller
Everyone happy !
Nobody has yet managed to explain why anyone would actually NEED Kinect 2 and not just the normal controller to do anything with Xbox One so why not make it optional?!?
For users, Kinect offers new ways to integrate with the system and Xbox Live above and beyond just using a controller. It offers new ways to recognise and interact with the users, with much more accuracy than the 360 Kinect. The user-identification features that Killer Instinct apparently features could just be the start of what could be possible.
For developers and publishers, Kinect will always be there. Without it, developers will be faced the question of whether to devote time and money for titles that can only be used by a portion of the user base. On the flip side, the fact that most titles won't be exclusive could mean that Kinect support (on those games) may be limited at best, unless Microsoft offer an incentive to those publishers.
For Microsoft, Kinect gives them several boons. One of these is advertising - Microsoft have applied for a patent that would allow Xbox One users to score achievements for watching specific videos or advertisements.
The benefit that Kinect2 offers for advertising and marketing efforts could be massive, and financially rewarding. Get enough data together, quality data that is gathered at the time rather than days or weeks after, and the data mining potential becomes staggering, even with the data anonymised. Smile at an advert? Kinect, if it can identify this as a 'smile', could record the fact that an advert received a positive response from a viewer. When you link this in with the push for TV integration, the potential for this side of things becomes a lot clearer. It could help the Xbox One pay for itself many months or years before it would otherwise do so.
You could say that there are more benefits for publishers, marketing and advertising concerns than there are for users.
I would prefer a Kinect-less SKU myself, or the choice of which SKU to go for, but I can't see it happening.