Though I am not an NX user, I do see the merit in having a separate forum because NX users do tend to be a close-knit and passionate bunch. That isn't a criticism, but I think it's a valid observation.
I think it would be EQUALLY valid to set up a separate forum for users of Adobe products (Photoshop, Lightroom, Elements) since there are a large number of Nikonians who are partial to these as well.
A third forum could be created as a catch-all for all other third party software (NiK, Alien Skin, Noise Ninja, etc).
As a forum user, I see a great advantage to the above. There would be less confusion and it would spur more useful discussion about each program without the inevitable "mine is better than yours" kinds of comparisons that happen in the current forum. Having just one forum for post-processing seems a bit limiting since post-processing is a large part of what we do with our cameras. There are different forums for different camera bodies, why not also for the different software we use?