Some very interesting thought experiments arise from this. Imagine these rare books are worth $5 million at auction.
$5 million can do a tremendous amount of good. It can feed a lot of people, educate a small village from birth to adulthood, save hundreds of lives from Malaria and other mosquito-born diseases, and pay for many other life-saving medical interventions.
Should the library sell the books and spend the money on the above items?
I know my analogy doesn't quite jive with the civilian-massacre-vs-book-thieving choice. But hopefully it shows just how immensely important libraries are. Had the Library at Alexandria not been destroyed, there's no telling how much earlier our civilization would have advanced to the relatively peaceful time we live in now.
I guess the difference between then and now is that we have the ability to digitize all these rare works. That should be one of the highest priorities for our civilization these days. Get them digitized and replicated. There's still the storage medium problems, but with enough replication, we can hopefully ensure the durability of all that information nonetheless.