I don't work on it because I don't want to reinvent email -- a trap many have tried -- but I do want explore ways of doing content-addressed distributed messaging, and a DCVS gives you those for free.
Exactly what I need: the opportunity to merge when trying to read my email.
In worldspace, every message's direct descendants form a DAG, i.e. classic threading, so messages only know about their ancestors or successors.
The "inbox" contains many independent, unrelated threads. From a client's perspective, I can see wanting to treat the inbox as a repo containing many files, each file being a thread.
From a protocol perspective, the total contents and state of the parties' inboxes is not relevant (nor particularly wise to share); but you would want to push your changeset to a thread (i.e. a new message) to your recipient. A git changeset can affect many files, which is not consistent with the semantics I'd want to convey.