There is also the very practical problem of efficiency: - Having to deal with an ML and a forum takes more time to deal with than if all posts would be in a single medium - Handling a large number of posts in a forum is much quicker than reading such a number of ML posts.
The second point became painfully evident when I came back from a week of vacation without internet and than found my telephone line cut by road works - ending up in a large backlog to deal with:
Opening emails one by one and sorting out all the quotes turns out to be a lengthy occupation. Subjects in the Mageia discuss ML are more corse-graind than thread titles in the forum: in the forum posts without interest are much quicker to skip, in the ML I frequently have to open posts that than turn out to have no interest for me. And the GUI of a the forum is more adequate for rapidly scrolling through a large amount of data, better than the standard email interface (partly due to the fact that forum posts use html, they normally are nicely structured and easy to just "look at") and ML posts llok quite often messy and take more careful reading.
For me the conclusion is clear: ML for general purpose communication, forum for the discussion of precise issues.
