Andreas Barth writes:
Of course, [git main always has features, sometimes lots of them, not in the distro packages] is true not only for mailman3, but also for exim, kde, gimp, whatever.
Well, this is more true of Mailman than those other packages for a few reasons. As Mark points out, Mailman is evolving, but I wouldn't say more rapidly than those other examples. However, one important driver of Mailman 3 evolution is that the Internet is a hostile environment, and many changes in Mailman are driven as a response to security or other issues that are out of your control as admin of an Internet host.
It's true that distros are pretty responsive to that kind of issue, and frequently backport security patches to the version they support quickly, but heavily patched versions have their issues, too.
In general I would say that distro packages are extremely well-suited to establishing a stable baseline for your host (and for Debian, at least, you can flexibly choose how stable various parts are), but for mission-critical applications you shouldn't *default* to the distro package. You should carefully evaluate whether the distro version historically has kept pace with upstream, whether it is featureful enough, and whether it's worth your time to do work that will eventually be done by the distro's package maintainer (you hope ;-).