If the community is split into two different camps, communication is going to suffer, plain and simple. I can receive email wherever I am and reply or not. I can set up filters and rules in my email program to sort the email posts. If the mails to the FlightGear-devel lists are archived in a forum, that's fine. If posts to a forum are likewise echoed to the -devel list, that's fine. In other words, if there are two ways to view a single communication venue, that's fine. But if there are two completely independent venues to discuss issues (say, the Users list), that would be very, very bad. It really defeats the purpose of having any kind of discussion group/list/forum at all.