I think compulsive hoarders would like it a lot. And especially the group of compulsive hoarders that also are pirates.
But I suspect that most normal ebook readers don't have as much interest in it.
Most normal ebook readers spend time reading books. The time spent getting books is small compared to the time finding out what book to read next and to actually reading it.
Personally I don't have any need for a OPDS client in Calibre. The OPDS client in my ebook reader app is enough. I only use it occasionally.