Now, I'm not a modder. I'm very from it, but I still hope I can contribute to the discussion with my viewpoint here. I'm a civ'ver, like anyone else for Civilization Revolution, Civilization IV, and Civilization V. I love the concept, and it's probably the top franchise in its niche, which is historical gaming. I probably play the game different from most other players, though, in that I really apply the games in the historical perspective.
I used World Builder a lot to create scenarios based on history, and each game I like roleplaying as a country, like the United States after the revolution or a colonial England. Now, obviously, Civilization Revolution didn't allow that, but that game got me interested enough to buy the already out Civilization IV and get me fully interested into this franchise. With Civilization IV, I just couldn't roleplay. The civics system was too simple, the tech tree was far too small and shallow, etc.
However, I found mods like something I believe is called New Realism, Rise of Mankind, and ultimately a New Dawn to satisfy the complexity I needed with Civilization to fit my particular style of gaming. The civics were complex enough, the tech tree made for more realism with how much depth there was, *there was a revolutions system*, etc. I had great fun with it. However, when Civilization V came out, it seemed much prettier at least, but I doubted it would fit my roleplaying style of gameplay.
As it turned out, though, after World Builder was released, I was surprised to see that, unlike Civilization IV, Civilization V had sufficient sophistication to play really as a historical game. The civics system is the primary reason, because there really isn't much that a mod could improve with it philosophically, unlike that incredibly simple civics system for Civilization IV. The Civ V civics system covers the variations and deviations there are in political science, like differing Liberty and Freedom, showing that the first societies had three pathways in kingdoms (Tradition), republics (Liberty), and simply being a warring state (Honor), etc. I would simply just add some more trees and more variations for titles when civic trees are mixed, since presently the titles for a leader, like Lord or Doge, are only for the civic tree last purchased. However, of course, the tech tree was as lackluster in Civilization IV.
Generally, Civilization V, I would say, is better off than Civilization IV was vanilla, but it will never be able to reach the quality or capacity, at least for me, as I had with my general Civilization IV experience, if it does not allow mods more leverage and liberty to design mods that can be the best fit for the civilization fans, because not everyone plays the game the same way, and you can't please everyone. Therefore, why not support the means to allow people to please themselves as much as possible?