clan1: that would be way too much to keep up with, and it would never be cleaned-up by the alliance's forum mod. so it would degenerate into a insult and cussing fest. With no mod, it would be practically useless with hundreds of posts within two weeks at best. I've moderated combined forums with over a thousand members( farsala had 2500 and zachy had about 1200), and its all you can do to keep up with the posts., even with a small percentage of those members actually posting.
The alliance Mod has to have Leader or Founder authority to create administrative tabs i.e. combined tab, and I've seen 23 alliances at once, but programming-wise, there has to be an upper limit to how many alliances can be added to a combined tab. That's alotta work to put that together and there would needs be a declaration on a minimum alliance membership in order to added. They would not do it for alliances with only a few members, unless those members had dozens of cities under their belt, in order to make it worthwhile for the mod.