A lot of people keep their mailboxes as archive of almost anything they do. Some of them like to have notifications even for their own actions (sent to their own mailboxes, obviously).
It shouldn't be turned on by default, but it would be nice to be able to opt-in.
It's one of features that I greatly miss in GitHub. I hate asymmetry of having only part of issue comments in mail and having to go to web to check full thread.
I guess it could be realized via filling CC: field, so there wouldn't be any need to conditionally prepare To: field depending on this option.