Actually, we're considering merging some forums; the reason that it was originally done was because one single forum couldn't handle the traffic, and the forum kept crashing. We have a lot more horsepower behind the forums though, and we definitely have a lot of flab here and there -- some forums are completely dead and should just be axed.
The reason we haven't done this since moving to our new server is primarily because (a) we were tired and (b) because we were considering jumping ship entirely to a different BB system such as vBulletin. Managing Forums in vB is a lot easier, and we'd save a lot of time merging forums in vB as opposed to doing it in UBB.
There's going to be major renovations taking place all across the network over the next few months: osx.macnn.com is being redesigned (well under way), reviews.macnn.com is going to be redesigned and updated (it's way obsolete now), we're going to clean up the forums by merging some and axing others, we're reviving AppleInsider, and we're likely going to set up an IRC chat server on one of our spare machines.
There's a lot of stuff going on right now, but I fully agree that the forums need a thorough rethinking.