As Nokthra said, differences in files between host-client will caues desyncs, which, if you're going to officially support modding, obviously has to be addressed. Some people will get xxx mods, some people will get yyy mods, 2 people will have zzz mods lol, so the game needs to be able to decide what to use, and that the clients have matching files.
It was one of the reasons we're doing the public overhaul mod, to give everyone a common "base" to work with.
