I am almost* entirely of the opinion that no book should ever be thrown away. Digitize if unwanted, but don't throw it away. I know that might require some jiggering of the IP laws, but knowledge has huge value and it really bums me out when books of knowledge are dumped. Even if the knowledge is antique, then the information about how the knowledge was perceived and transmitted becomes valuable to later generations.
I'm a librarian. I became a librarian because I've always loved libraries and always loved books. I haven't met many librarians who didn't have the same formative experience and who don't feel the same way.
Counter to what the post's author implies, almost all librarians hate destroying books[0]. But we have to do it almost every day, because most libraries accept donations and only a very small fraction of these donated books are appropriate for the collection[1]. Many libraries have book sales, or sell donated or weeded books online, either directly or through partners like Better World Books[2]. We quickly learn that some copies of some books, either because of their poor condition or because no one is interested in reading them, have to be recycled or destroyed. It's no fun, but there are no alternatives[3].
As the post's author mentions in the CREW discussion in her post, librarians make an enormous distinction between the 5,000th copy of a book that is held in some library collection somewhere[4] and the last few copies. It's possible that San Francisco Public Library knowingly destroyed the last copy, or one of the last known copies, of a title, or even multiple titles, but I would be surprised if that was a policy rather than a mistake. In general, libraries either have a place to store such copies or can find another library that will add it to its collection and put it into circulation or storage.
[0] http://www.inthelibrarywiththeleadpipe.org/2013/killing-sir-...
[1] By appropriate, I'm talking about the book's physical condition and the likelihood that it will circulate enough to justify processing it and putting it on the shelf, because it is competing for that shelf space with thousands of other donations, plus the ~1M new books per year that we could buy and put on the shelf in its place.
[2] http://www.betterworldbooks.com/
[3] Unfortunately, there are only so many places to donate books, and we give away as many as we can, and then some, to places that are interested in receiving donations.
[4] https://www.worldcat.org/ is one place we look, though OCLC makes it prohibitively expensive for most small libraries to make their collections available in WorldCat: http://www.inthelibrarywiththeleadpipe.org/2008/a-useful-amp...
Goodness, I am not saying that you want to. There's a limit on shelf space in the world. I'm just noting that I'd rather see books digitized than dumped.
http://awfullibrarybooks.net/ is a blog edited by two librarians, showing books weeded by them or their readers. It's hard to argue against their choices: thirty-year-old romance novels? An autobiography by a 1980s football player? "Dress Like Liberace"? There are plenty of books that need to be thrown away.
Now imagine you're in 2140, reading that 1980s football player's autobiography: almost every page would contain something you never realized about the past. An etiquette book from 1832, for example, is far more valuable to us now than it was in 1852.
There is certainly value in keeping some copies around for future historians. However not every local library needs to or can afford to, and they aren't of much value to the people who use the library.
That's perfectly correct, and a sensible response. The comment I replied to, however, was simply saying that those books need to die, which is much less sensible.
* There are some really bad books out there.