In Gmail, right now, I have 65,290 (non spam) messages, stretching back over 3 years. I routinely (once a week or so) need to refer to something that is at least six months old. I just had to dug in old backups to find an instant message log from 2006, because I had some information there that could not be found anywhere else.
My take on that is that yes, history matters, a lot. Moreover, being able to get to that history matters as well.
Today, I cannot imagine how I would try to manage myself without Gmail there to help me, being able to efficiently & easily search my entire history is the number one reason I love it so much.
Oh, and also important, make sure you have the backups :-) That IM log that I was talking about survived about 4 different computer transfers.