I think that the API is, for the moment at least, more likely to be used for custom work rather than mainstream plugins. It's the nature of the beast.

Being incomplete, the REST API isn't really ready for prime time yet, and potentially is a bit of a moving target. Development today is potentially tomorrow's support nightmare if any of the functionality changes. Most people who have developed against this sort of new tool expect it, and the smart ones generally stay away until it's a couple of version up the road.

IMHO, REST API should have remained as an optional plug in rather than being part of the core, at least until it is completed and fully functional. Jamming it in there now is just not in the best interests of the core right now.