So because I'm almost sick and got this idea into my head, I went through my gmail archive culling stuff.
The final body count: from 22000+ conversations down to less than 6000. All deleted in blocks of 20, which seems to be the only way that gmail will show you search results. And that was a lot of clicking, let me tell you. Probably about two hours of work over two nights.
Most of it was archival stuff from when I still thought email lists were a good idea. CentOS lists, several versions of RedHat lists, Cobbler and Kickstart lists, even several years of autsupott (the Ottawa support list for families with Autism). None of which I would ever want to see again. There was also a metric ton of old spam reports, cron barfs, root log reports from several systems long gone by, and the inevitable nonsense from our good old friend Mailer Daemon.
All gone. My mailbox now occupies only 290M of stuff instead of the several GB it did before the clean up.
I mean, sure you can keep all this stuff, disk is cheap -- and in the case of gmail, largely free -- but if you can't find it and never look at it, what's the point?
Besides, now Gmail disk space is shared with Google Drive space, and I actually *use* that.