With communications technology it is not really helpful to focus on time like that.
Metcalfe's law tells us that the value of a communications network is proportional to the number of users squared.
If you have a Discourse site with 50 users, the value is 50^2 = 2,500
Now take the Metcalfe value of all the free software communities happily communicating over Mailman and SmartList using their email addresses. If you assume there are 50,000 participants in the universe of loosely coupled email lists and communities, then the Metcalfe value = 50,000^2 = 2,500,000,000.
A new community joining this loose network actually becomes part of that: in fact, if you add the 50 people from that Discourse example to the other 50,000 then you get 50,050^2 = 2,505,002,500. Notice that this is much bigger than the sum of two squares: communities gain a lot more by working together. This also makes the community more robust against competition from closed, proprietary alternatives.
If Discourse plays happily in the world of email then it also becomes part of that success story.
Another unpleasant thing I noticed in emails from another Discourse site is that the URLs in the emails don't go back to the domain of the Discourse site, they all seem to go through some unrelated redirection service. That made me more suspicious of using Discourse or communities who rely on it.