Oh, I, for one, certainly would be. It really wouldn't be hard to upload whatever images we needed the normal way, if ever we coveted an image that was already on Wikipedia. And as you say, that's an incredibly rare situation to begin with.
But the thing is, I'm not sure this is possible, on the coding level. The way I heard it told, Wikia defaulting to Wikipedia files when linked to a filename that doesn't exist within the particular Wiki's image library is simply a feature of the root code — not something individual Wikis can turn on and off on their own accord.