[Re SimCity 4 and RoR modding] Both are utterly ridiculous situations, especially so in the first case. Where the end user had to hunt all over the internet for over 500 separate files because the people who make the models won't let the people who make the installers put the models in the installers, and only make them available as a single building download.
Both open content licenses I am advocating in my previous post would have prevented that.
CC BY-NC-SA allows modders to republish the tweaked mod as a whole, because there's no need to keep the original file intact. This is the most convenient option for the players. (There'll be an attribution notice someplace that lists whose work went into this mod.) If you want to see the original mod, you need to find it on the web. (Shouldn't be difficult.)
CC BY-ND requires to keep the original mod intact, so can only distribute a patch (aka "install my tweak after you have installed this"), but it allows you to distribute all prerequisites (i.e. the mods you are patching) along with your tweak. This is less convenient, but it makes sure that the players can still install the original mod by itself after they have downloaded the tweaked version. Thus, players (and other modders) always have access to the original modders "pure" art.