@pleonasm - Well, it's not only old books that are valuable. Take the Lost Girls book by Alan Moore. He couldn't find a publisher for it, so he and his wife did it together and there aren't a huge amount of copies. It's sexually explicit so many bookstores won't carry it and those that do can charge up to $100 for it.
Whether or not you agree that the story is worth reading, it's a valuable item nonetheless and will only increase in value, no doubt. Treating it cavalierly seems just as much of a mistake as treating a pair of diamond earrings badly.
And the most valuable books are that way because they have history and a story of their own. They aren't just a copy of Pride and Prejudice, they are the copy that Jane gave to her mother (for example). Owning something like that would be like owning history and I would love to be able to do it.