One of the biggest uses for blogging or tweeting is to show that there is a person there as part of the institution to interact with. When a user is faced with solely a catalogue they’re dealing with a collection of items, be they journal articles databases exhibits or books (which I hear do still exist). When you include some sort of dynamic content that’s been made by a person, you’re reminding the user that there are people behind these services.
Example: The Powerhouse Museum in Sydney (Australia – it’s where I’m on my co-op so you’re going to get antipodean examples) is using Koha as their ILS. Integrating their library blog onto the main page of the OPAC makes the catalogue a destination for users. And then when the librarian is blogging about something in their collection (and they’ve got some cool stuff) and deep-links to it, that’s giving the user examples of how people can interact with the catalogue.
Having a personality that reminds people The Library isn’t some building but a collection of librarians is important, and not only when budgets are being threatened. Users are more likely to engage with you if they know there’s someone to engage with.