Here's another toughie. What material on the internet should be preserved?

Sparked by a discussion on NPR. There is a conference going on right now at the Library of Congress about archiving material from the internet. They are considering archiving tweets, Craiglists posts, etc. as well as website content. Is this project worth doing? Are we belaboring the future with our ephemera or is it all worthwhile? Parenthetically, who should pay for such archiving and how do we know the storage technology will continue to exist? Your thoughts?