Saving pages is not the primary use case for the image meta data portion - users tapping on an image and not having to wait for another server round trip to determine the high res url and meta data is the primary use case.
Having all this data as part of one request means image taps can be much lower latency from the user's perspective. The panel can pop up immediately with meta data overlay over the already downloaded low res image and can immediately begin lazy loading the high res version. It will seem much more responsive, which is really important.
As a side benefit, saving page data for offline access also becomes easier, but users tap on images (I think) more than they save pages.
Saving a page for offline is not that speed sensitive, however, serving this information as part of normal page views would definitely impact overall latency.