For me, it's not just the ease of finding data or people when needed, but there is something both funny and embarrassing about dredging up messages I sent years ago to people I haven't spoken to in years, now seeing the event in 20/20 hindsight, or simply noticing the way I have adapted my writing style from then until now. As time passes by, people do change, yet I have point-in-time snapshots of myself, friends, family and colleagues that will not be changed or edited for history. With the right keyword searches, I can cringe at my struggles with finances in college, wince at attempts to gain somebody's attention, or root myself on as I look back at challenges and changes in their infancy. I can also run a quick search to remind somebody of what they've said or use an old conversation to make a point.
What I have, using Apple Mail and the Mac's built-in Spotlight functionality, combined with a pack-rat like attention to e-mail storage, is what Google is hoping the world will move to with its integration of GMail and Google Desktop. You should never have to delete an e-mail, and you should be able to find any message with a well-defined search and parameters. While this is the future for some, for me, we're already there, and in case you were curious, the few times I need the data, its interesting, but not revolutionary. Fully 90% of all my archived e-mail will never be seen again. The good news is that it doesn't take up any physical space - only theoretical bytes from a hard drive with plenty of headroom.