The more you seperate the forums for different series, and hence the communities, the easier it is for those who want to be 'purists' on one game or another. As far as someone who solely plays Sonic is concerned, the forums they visit would just be for Sonic, and likewise for the others when irrelevant forums are hidden.
The more you integrate the forums and communities, the easier it is for those who just have a casual interest in a bit of everything to waver from one place to the next. This is probably why I voted for greater integration, because otherwise I will probably stay solely on the Sonic or central type forums, if doing otherwise would mean going through a whole other site.
At the very least I am pro some shared Welcome Center and Lounge and perhaps staff action, if not shared then a 'central' one that is not series affiliated. You could then have series-affiliated general forums (oxymoronic?), but if you have only that then I feel it is somewhat encouraging segregation. I'm not saying you have to get on with people who compete in other things, I'm just saying the option should be more open.