It's a stupid interface, imo, in dire need of some user interface TESTING -- because anyone who did a bit of testing would have said:
1) show a way to FILTER the Media Library so it ONLY shows those images that are associated with THIS post (or page)
2) provide a way to select images from the entire Media Library to ADD as a gallery to this post/page
3) provide a method to create a GLOBALLY accessible image gallery that can be selected in ANY post/page for insertion
otherwise, you end up with what we have: a confusing display of many files and images that you can't tell if they are duplicated or not or whether they are even being used or not.
Rather pathetic, given all the requests for a more useful and non-buggy function for uploading/inserting/deleting images and uploaded files.
Sorry if that sounds harsh, but it feels like the basics of user-friendly interface design were just ignored in order to use the flashy ajax thingies... from missing functionality to the ever-annoying "close if you click anywhere" lightbox/thickbox? issue that often causes users to repeat an upload because they either aren't sure if it was completed or not...