I think that hitting "reply" should still contain a pointer even if you don't include the @username. Doesn't the threaded-reply already imply that you're addressing that person?
I think that would require a tweak to the UI to make it more obvious that a tweet is a reply. There's already some subtle stuff there (small-font "in reply to" and I think a little curved arrow), but the most obvious part that the content starts with "@".
