A casual game differentiates from a quick game in more ways than "more time." Turns are plotted by calendar days not just a few hours. This poses logistical problems as well as others.
Planning for something like that consists of more than just opening a game and advertising in room, global, or clan chat. One thing a casual forum would do is have a place for games to be advertised and promoted, and further for discussion of the game to be held. Further as a game progresses its commonplace that people will timeout of a game or become afk by skipping turns and the host is either unaware or has to be personally messaged to appeal to him kicking the player. On a forum with a stronger community where the game was advertised to begin with, it provides a place for players to engage in a way similar to chat in quick games. Strategy also, is complicated in a casual game. One must account for the fact that the opening turn is much different than in quick, and that country selection has to be much more strategic since another user can capture a country before you even select it. Additionally through the progression of a game, things are *much* more strategically minded. To me this adds a fine art to the game not achievable through a quick game.
Making such a forum is by no means splitting up the community. Instead it provides a steady and slow board to facilitate the steady and slow nature of casual games.
Have you ever even played a casual game? Maybe you think its an insignificant community that can be ignored because you aren't a part of it and haven't noticed consistent problems which could be alleviated by a more connected community. It's more than just joining a 50k. Ultimately what I think would come from a casual forum is a stronger casual community, one which is not so small (though you should look to see *just how many* casual games are open).
And by your reasoning of this being an invalid sub-forum, that may also apply to other boards on the forum which you don't seem to protest against since they have been there for so long
Maybe try a few casual games before you say a casual forum is a stupid idea. Part of why I made four choices instead of two is to demonstrate a gap in the interests in casual players and quick-only ones. I've not seen anyone yet vote no who plays casual but I'd be very interested in what they would say about why.