It's a downside of the engine - to store the most recently used files in the client folder. If it would be possible to store them on the server side, it would affect the prefomace in a bad way (and it require a really good machine and connection). Moreover, as Luther said, you can do this with 3D models, but you can do the same thing (and some more things, such as deleting certain tiles, which prevent you to see some hidden items) with 2D as well. I believe that some feature that's checking at least basic files to stay original have to be added at the later stage of development, because if it wouldn't, some players using this "feature" will get an advantage.