Indeed, we're shipping the first half of the core REST API with 4.4 (just about to hit RC), and the rest in the next release.
In the same vein, the wordpress.com api does indeed appear to be rather well documented -- shouldn't it be rather easy to implement enough of that to allow using this new web-app for interfacing with it? (Even if most of the calls just return 403 or 500 - presumably the app handles that somewhat gracefully)?
(There are some things in the WP.com API that don't make sense for self-hosted sites, and vice versa.)
The list is long: http://jetpack.me/features/ (probably half of those require the connection)
Examples include stats (a pain to scale on shared hosting), related posts (also known to kill database servers), site uptime monitoring, image CDN (including on the fly thumbnailing), and so on.
It basically provides the things that you probably don't want to self-host. You get some of the scaling of WordPress.com while being able to do your own thing (plugins, custom theme, etc.).