If the forums were combined, it wouldn't really changing my reading/browsing experience, because I always click the New Posts link in the toolbar and then click on all of the thread titles that interest me. I don't really notice which forum each of the threads is in.
For members who don't read the boards that way, I think separate boards probably keep things feeling tidy & manageable. A combined forum would mean they'd have to sort through a much busier, faster-moving list of threads, and discussions normally found on the FFD might get drowned out by the more prolific posting of MFD-type threads.
Just my 2¢.