Personally, I often refer back to emails of what people sent me in order to have proper context, both technical, logistical, etc. Instructions on how to do things, how we debugged something or other, etc. That makes me more productive and useful as well as other people. If we gotta write every email over again and put it up on a wiki, that just won't happen.

Write it on the wiki first then just email the link to the wiki. No additional effort and then the guy who joins two years after everyone on the email chain has left and now has to maintain it also has that info.

I meant specifically, things like "Wait, who is this person? What did they want again? What did I tell them last time?" That's not stuff you put on a wiki.

This may or may not work for you, but I keep notes in a collection of Google docs.

Depending on context, these are either shared with the other person (and usually editable by them) or is accessible only by me.

At the volume of mail I was receiving at Google (100+/day), there's just no time to manually index things like that. Why can't we let email archives just function?

If you need it for your workflow, it is very easy to avoid the auto-deletes. I know plenty of people who blanket keep all their old emails without a problem.