In reply to Nick Halsey.

@Nick, What do you mean "there is no need"? Of course there will be a need for all the themes that are interested in being on wordpress.com, or show "jetpack compatibility", and once many themes will support it, they will start crying about the fact that it is something common to most themes, and why can't it be in core instead of forcing them to implement the same functionality again and again in every new theme. And before you know it, it will become part of core.
This was exactly how site logo became part of core, although it is easy to be implemented by theme authors.

And that is of course not the only example. Isn't your css editing feature that you want to push into 4.7 an exact rip off of a jetpack feature? Obviously nothing wrong with adopting ideas and even code from other places, but it seems like what @Jermey implies is correct - whatever goes into jetpack will end up in core with much less scrutiny (as it is already installed on millions of sites).