I guess I'm a bit of a hoarder by nature. I don't like to toss things that might have value one day. But I'm okay with the occasional housecleaning.
However, when it comes to records and library-type archives, I'm with SlimBoyFat. I often wonder about these treasured letters that we see on occasion from people who became famous after their death - what prompted someone to keep them instead of tossing them? They have become an important historical archive, yet at the time (and perhaps for many years after) they probably had no apparent value to anyone. But we are sure glad someone kept them. So where do you draw the line?
As jgnat says, there is a lot of trash in libraries. But I've also seen obscure "trashy" early works by authors, artists, etc. become important parts of the whole picture. What if someone had tossed them? I'll use an example of a project I'm currently working on - archiving the political record of my province. They are trying to put all the bills online, and have managed to find most of them - but the very first bill ever passed here is gone. No one bothered to keep it. It wasn't important at the time. It sure is now. In my research I've come across all kinds of things that it would be important and valuable to help us know more of the story - letters, stories, commentaries. But it's all been trashed and is gone.
When it comes to thoughts, opinions, personal chronicles of growth, etc. - who is to say what matters and what doesn't? You may no longer be friends with someone so you throw out all their correspondence. What if that friend was Shakespeare?
I just scanned the topics here, and already I can see a very different board. I don't see any topics dealing with fun, humour, lightness. I'm guessing a lot of this was deemed fluff. But was it, really? Whether you agree or not, this board is a community, and a community does all kinds of things together - laugh, cry, play games, and discuss serious stuff. What happens when you edit all that out after the fact?
With forums a whole other ethical issue arises. If you start to delete threads, what about links? Then do you delete posters who are no longer around? What about loss of continuity? Do you start editing threads so they make sense? Then what happens to intent and meaning of those who posted originally? ARe they now left with bits and pieces of a conversation that says something quite different than intended? Stuff that is on the Internet forever that no longer reflects original intent? It's complicated. And how do you judge which pieces should be kept or discarded? I know I feel differently about things on any given day, and I've often regretted things I've tossed.
I guess when it comes to the Internet the rules around saving information will continue to evolve, but I'm still on the side that says keep it all.