Wow, I started this topic 10 years ago, almost to the day! 
My personal feelings about this idea have not changed. If it existed, I would enable it by default for all users on two private sites I manage. What I do on those sites for myself is create a second account that I use to just receive email summaries.
I agree the weekly summaries are 
gorgeous and do a great job summarizing what’s been happening, all in a nicely curated package. They likely would help to draw members into more discussions by letting them know what’s going on across the site. They also would make participating in a given forum a bit easier, because members would not have to watch quite so many categories and topics - they wouldn’t miss anything. Also, site owners could rely on these email summaries to reach all members with announcements and so wouldn’t have to set up announcements categories that every member is watching by default.
(Do note that since 2014, the concept of “email digest” has evolved into “email summary”. I don’t know exactly how it’s put together, but the email summary is not comprehensive and just shows a selection of activity.)
I don’t actually know why @codinghorror was against adding this as a user preference - he never gave any reasons why back then. My suspicion is that we want to encourage people to come participate on the forum rather than sitting back and waiting for email summaries to show up in their inboxes for them to read. There may also be some technical/performance reasons. Email summaries are personalized to the member which takes server resources, and sending them out might inundate servers delivering transactional email.
There has also been quite a culture shift in the last ten years. Many people don’t even use email anymore!
There’s a related topic Set "default email digest frequency" to 3-4 days about changing the duration of the email summary to 3 or 4 days, and also being able to set the days when they are sent. I suspect that change (in addition to sending to everyone) could really go far to increase participation because suddenly you’d see alot of people logging in at the same time!