Seriously though, those services are fine on a powerful tower PC plugged into the wall, but if you're on the move on battery power, they are unbearable.
For example, the Elm community has both. The Discourse forum is technical and business-only yet there's a clean record of these discussions. The Slack chat is where I hang out, get to know people, and participate in more relaxed chit chat about Elm, webdev, and building applications.
Elm used to just have a mailing list but it was obsoleted and shut down with the creation of the Slack group and Discourse forum which were far more popular.
Any community that only has a mailing list could benefit from experimenting with a proper forum. I've seen this experiment broaden a community time and time again as you move away from only selecting for the type of person who likes mailing lists. And notice that HN isn't a mailing list either.
For example, I would imagine that the sort of people interested in high-quality ebooks extend beyond mailing list loving super-techies. Even a subreddit would be a nice option.
Some people might say it's useful and others might say it encourages people to comment first and possibly focus on polishing the comment second.
E-mail sucks.
It's not very full-featured yet, but it's so fantastic I want to pay for it. It's a shame the companies themselves don't offer clients like it.
Also I'm prefer to avoid Google services 'cause privacy issues.