I find it interesting seeing how few people understand the point of themes like this.

Performance is always important, and theoretically themes like this will always outperform and outscale something which has to do a complete page render, both client and server side each time. Even if the HTML is cached, it's still going to be faster to just grab some cached JSON than to haul in a complete dump of HTML.

As to whether it's a good idea to double up on templates and create a huge increase in complexity ... well that's a different question altogether, and I guess somewhat dependent on the particular project in question. I can only think of a few of projects I've ever worked on which I think would have been suitable for this sort of treatment.

I think the double template issues inherent in this type of theme will be key to Node JS's thrust to dominate server side development in future.

Perhaps this is the beginning of the end for WordPress's dominance of the web publishing?