So tricky... IMO the best think to do is to wait for public usage of the API.
I'm working all the days with distributing Telecom APIs for public or paid usages and believe me, you can not make any good development (that works in all the cases) based on those APIs until they are stable.
In the case of the WP API the result for the users is that they'll think that your plugin is so bugged... Just because of the WPREST API is not finalized... And as Nate Wright said.. You must keep compatability with previous releases if you are some....
Not so good for business.

It is like future CSS4 or 5...Nobody will use it in production as porting are more or less implemented on all browsers ..
For the moment it is just for testing purposes...

But in // we can all use them for mockups, just to be ready when the D Day will arrive...