Just a quick note:  I totally agree that the value of any item is socially constructed rather than inherent.  That doesn't mean that as librarians we can't add our voices to the conversation and contribute our perspective if we believe that something is important, but it does mean that knowing our communities is just as important as knowing our collections.  If we want to end up as repositories of obsolete junk, shutting ourselves off and disengaging from dialogue with the social context of our collections is a great way to do it.