Personally, I prefer typing BBcode than using a WYSIWYG editor. It goes faster for me.
On the other hand: I understand the wish for it from a newby point of view. For instance on a forum of mine where there are many users with almost no knowledge about code whatsoever, they simply don't use formatting since they don't know how. For that kind of people a WYSIWYG editor would be great.
Besides that, there's the point of usebillity. I think it's not very user-frindly to require knowledge of some code or extra mouseclicks to create a nice formatted message. The argument about "it's a forum, not a website" is wrong IMHO; there are boards out there wich have many large and complex messages. One forum of mine, about science ("wetenschapsforum" or translated "science forum") has many messages where complex theories are explained. Easy formatting is very welcome there. We would like to make that formatting as easy as possible since we'd rather see the users gain scientific knowledge than learning some BBcode. For now there is no good WYSIWYG editor for that board (IPB, not phpBB) since it would break on stuff like LaTeX, WebGraph of chemical formulas.
And that's the main issue for me this moment: With my current knowledge it's impossible to create a descent WYSIWYG editor wich copes like all kind of custom BBcode, especially server sided BBcodes. Perhaps some kind of AJAX thingy can handle that, but my expierence with AJAX is very little.
Above message may contain errors in grammar, spelling or wrongly chosen words. This is because English isn't my mother tongue. My apologies in advance.