I don’t really understand why libraries wouldn’t use RSS. Maybe if your library doesn’t do anything you want to keep people updated on?
A really basic use for RSS (and Twitter) is to have a constant stream of interesting things past your eyes. For a library, spotting the things in those streams that might be useful for passing on to their users is great. It shows they aren’t siloed off in their own little world of books, and acknowledges that the world of the internet is part of the library. Even something as simple as subscribing to feeds from your user groups’ websites to know about the events they’re having is a big deal. That’s how you become part of the community. You can’t rely on people coming into the library to tell you things. You have to be proactive and give the users a way to shape their own experiences with your institution.
An example of things librarians can do to make their catalogues more accessible with RSS is to make it easy to set up RSS feeds of OPAC searches or tags or lists. If as a user I come to the site looking for books with strong female protagonists and I find that the users have tagged a pile of great books with “badass heroines”, if I can subscribe to that tag cloud those books show up in my feedreader that’d be sweet.
RSS can push the library out into people’s consciousness without requiring them to come all the way to the site to see what’s new. People are lazy and if they can get the information they want given to them in the way they want it, that’s great.