What was the giant win for implementing this nexus of confusion?
The reasoning is that a forum is a place to discuss ideas and such discussions can only be enriched by a wider and more diverse audience.
Personally I can understand and partially agree with this argument but I do think there are a few flies in the ointment. First off most of the sites have a specific purpose. People that go to a specific site go there for that purpose and significant amount of "Off Topic" discussion dilutes the benefit of the site. Also as has been noted these types of topics tend to generate some amount of grief and can tend to scare off new users.
However, you could also argue that someone that plays Sins might be interested in Elemental, Demigod and GalCiv2 and so some amount of "cross posting" is both beneficial as well as good advertising.
All in all the mixing of the forums have significantly changed the dynamics of the site that I visit most (galciv2) and in my opinion the changes have not been for the better.
From Stardock's point of view perhaps the commonality makes supporting them easier, I'm not sure. In any case this has been discussed and it's doubtful that this will change and the likelihood is that there will be more sharing in the future rather than less.
Given all this I think most folks accept the situation, it's not like there's anything we can do about it. The one thing that I think would be very helpful is if the poster's site was known for each reply. At least then you would have some idea as to the context of someone's reply.
I'm also pretty sure that it's not really necessary to have close to 400 unique forums duplicated across sites at different levels of the hierarchy on each site. A pruning of the forums along with a published map of them would go a long way to helping folks make sense of the system. Otherwise you really don't have a clue.