This comes up from time to time, so below is the reply from the team unless things have changed in codinghorrorland. I still think it would be a valuable user setting and would enable it myself if it were available - people like to catch up on what they are missing via digests even when they have been logging in.
In our community we solved this with a monthly member newsletter, which we call “discussion highlights”. It’s basically a copy-paste job from the digest preview admin page right now, but we are considering upgrading it into a fully fledged structured member newsletter covering more than just discourse discussions.