Picking up on your last sentence. Certainly when it comes to Archives and University Special Collections, a Researcher is after material related to a specific topic/area/person/etc. So for us (a University), putting a very items up online *really* *well* if of little use when the researcher (who could be anywhere in the world) is after something else in the collection.

It will be different with Art sure, but I imagine the undergrad in Brazil trying to do their dissertation on some less known works from an artist, let us say one of which is held in a UK museum. Being able to access one of those art works online when they have no local copy of the work, I presume, must be incredibly useful. Though this seems argue for the long tail of works being put online. I guess it is a very different use case from a random person who just wants to browse some collections.