This might sound kinda silly, but I've never written a book IG, and I don't know how.... So, for me, it would take a little while to figure out how to leave a note on the message board.
And also, it does take out one aspect of the Forum Thread RPs. As it stands now, we can reply to previous posts, by having our characters come up, and either defile, or scribble or do any number of things in response to previous messages. On the In Game boards, it might look a little silly to have a second book posted beneath the first one saying "The note above me now has a dagger sticking through it." And being able to play back and forth with people on the forum threads was something I always enjoyed.
I really like the concept of using both, as it allows for the RP interactions, as well as letting game members who might not use the forums a chance to see the notices. Is there really a good reason for not using both? I think as long as players make an effort to put concurrent posts on both the forum and the game boards, it shouldn't be a big deal.