If the user hasn't input anything, a draft isn't saved. With digests, I think it's more important to have the reply window default to open than with individual messages. With an individual message, it's easy to reply within the email program. You can't do that with a digest. So I'd venture to guess that the majority of people clicking that link in digests, are doing so to reply.
I use the link from the html digest when I want to hit the Like button, which saves an empty draft. It would be better if a draft wasn't saved when the user hasn't input anything.
