I think, the points about search and archival must apply to any communications medium. If one isn't searching through their IM history but does so for emails,

I think, the points about search and archival must apply to any communications medium. If one isn't searching through their IM history but does so for emails, I guess, it should mean they just don't send anything non-ephemeral via IM. (May be this is because IM has pool usability for any large bodies of text, while email handles those really well.)