I always believed this was part of the goal of wave, to integrate email into a more real time setting, and was always surprised that email integration never materialised.
I think the obvious advantage of doing it over email as opposed to instant messaging / twitter is that email is ubiqutous, I dont know exactly how well the worlds email servers would handle it though :)
I think an interesting alternative, instead of bringing the advantages of instant messaging to email is to bring the advantages of email to instant messaging, threads are ad hoc rooms that people can join / leave, the server can persist message etc (I just realised I am basically saying I want wave, but with email integration and without a sucky client)