I suppose the example is a little artificial. It isn't really for using some of the JSON response while ignoring the rest (well, you can use it for that but it isn't the main use).

I got the idea for this project working on data vis. Not all of the data was visible and we wanted to display the first bit of data quicker without waiting for all of it to arrive. We could have just sent the visible bit but it was good to have some data ready in the off-screen section for when the user scrolled.

Before that I worked on a service where we were aggregating 6 or 7 services into a single JSON. Some of the services were quicker than others but because the AJAX lib we were using waited for the whole response they all had to go at the speed of the slowest component.

We could have done multiple requests but it was more elegant to serve a whole page's json in one call. Also, we cached the slow services so they were only sometimes slow.