I generally agree with Josh above. Until core has a proper dependency system, these are dangerous waters to venture into. However, I believe this is a good move for several reasons, some to do with the REST API and others to do with making sure theme authors know that we're open now (and have always been) to innovative ideas.

Any theme authors using the plugin should at least warn users that they don't know what the future holds.

This is a temporary measure to see if we can get some more eyes on the REST API. The more people working with it, the better it'll be and the more likely it'll get pushed into core.

As I've said elsewhere, I seriously doubt we'll see many, if any, theme authors take advantage of this opportunity. There are a few reasons for this. Most of them don't even know that much about the REST API. Only a fraction of them understand what this means for theme development. And, an even smaller percentage understand the documentation enough to use it.