One of my pet theories when I developed intercooler.js (http://intercoolerjs.org/) was that, by targeting specific UI elements with only the data necessary, you might actually cut down on the amount of data transfer between the client and server when compared with some general JSON APIs, despite the fact that HTML is a less efficient data format.

I'd expect this to hold, in particular, in areas where JSON isn't particularly a particularly efficient encoding mechanism (e.g. tables)

It's an interesting thing to consider, at least.