Some groups have also always existed in their own forums and had little to do with this place even in its heyday. Quarter to Three and Something Awful come to mind.
As far as interest in other communities goes,
you can start by taking a look here. Those are the ones known to me (by name only, most of them).
It should be noted that there seems to be relatively more community cross pollination at this time, with folks from all over wandering to the Desura forum and some traffic going the other way too.
Each of these communities has its place, its own preferences and each is the preferred haunt of someone for whatever reason. For example, I'm a moderator of this community, run the one on Desura and am a member of both Dom3Mods and Qt3. Doubtless there are other communities not on that list that I've never heard of, but that's neither here nor there.
As long as everyone adheres to the rule of keeping the business of one forum the business of that forum and doesn't bring baggage with them, there is no reason to kick a fuss over it. We (Dominions players in general) are a small enough community collectively that we don't need to borrow trouble.
Of all the communities I've seen, I've not seen attempts at killing off the others, as if that were possible in the first place. The most conflict has come out of some of the differences between this forum and some of its current and former members due to disagreements over certain policies of Shrapnel Games.
That horse has been well and truly beaten to death, resurrected, beaten to death again, the corpse reanimated and then once more beaten back down, so I'd appreciate it if was left to lie where it's fallen. There is absolutely zero reason to bring those things up again and up until now these old issues have been remarkably quiescent. Please let them be.