I like the forums split for the following reasons*
1) Keeps topics from rotating off the front page
(I'll admit, I hardly ever look back the first page of a forum unless there is something specific I am looking for, that, plus, I think back to long ago when there were fewer forums on this board, I remember seeing people "bump" their threads a lot to keep them on the front page)
2) Keeps 'cross talk' down
(back before the subforum split for different editions, all too often i would see a dozen vocal people who would just post back and forth about one edition over the other... I don't want to call it 'edition waring' because really it was usually the same dozen people derailing the threads and ignoring them avoids seeing their posts, but the threads get derailed to the point of silence by that point)
3) it makes it easier when reading/answering rules-related questions...
(if someone new just jumps in and says "i'm new here, how does damage resistance work?" it's a lot easier to see whether they are posting in a pathfinder/3.5/ogl or 4e discussion forum to know how to answer the question on the basis of the little info the OP actually gave)
4) it allows me to be selective
(i just subscribe to the forums that interest me and just focus on those and thus don't need to worry about the traffic related to other things. as an example, i like to visit the media forum. ever since it got merged with off topic stuff, i find myself less excited to go to that particular forum since the noise-to-post ratio -- from my perspective -- has increased. disclaimer: i completely understand the reasoning behind that merger, i'm not picking on the decision, just using it as an example from personal perspective.)
*all these reasons are, of course, subject to my own whim and are in means meant to represent anyone beyond myself.
Having said that, i can see the advantages of it being combined. Would I personally be upset/hate it/etc if the general forums were merged together to forum super forum? na.
it would just increase what I perceive the noise-to-information ratio. And, hey, i might see something mentioned for a game system I haven't tried that would make a good addition to the game i am playing with some modifications. Or, for people who have site-time load issues one forum is easier to stomach than many. And, as you said, it would show as more activity to those who don't bother going to the subforums like I do.
To put it bluntly, I think you are in one of those situations where you do it one way and group A is vocal in opposition, so you adjust and do it the other way and you'll have group B in vocal opposition. I think you (Morrus) yourself once said that if you keep trying to please everyone at once with the boards you end up pleasing no one (at least I think it was you, may have been back at Eric's days, or one of the mods making a retort to someone else's comments... or maybe it was a movie or book or show or song and I am just a senile old man).
Or maybe find a way to get thread tags to replicate forum delineation (though I understand tag usage is not necessarily an easy thing to get people indoctrinated into)
Maybe the general forum should instead read like an index that pulls from all the other forums (I believe you have something like this on the front page.. or at least used to for a couple weeks while testing other stuff -- it acts like an index of all the recent threads). Though, I don't know how practical that would be in usage since someone couldn't just go from thread to thread, they'd have to keep coming back to the general index.
anyway, just my late night rambles before i go to bed. apologies if my sleepy-thoughts are not entirely coherent.