Also, there's one other difference versus a true Mod:
> You can't NOT use it.
Mods can be turned off and on at will; you can play one game with my mod, then a game with Thal's mods, then an unmodded game, and back again without any conflicts. But if you're changing the core files, then this isn't really an option; you can revert your changes by getting Steam to fix the files, but there's no in-between. And if the file you've changed conflicts with what a mod you're using is trying to do, then it can break the game in a variety of ways.
Now, if the change in question is purely additive (adding a new asset definition to an existing file, like for custom sounds), then it's not so bad; the vanilla game would still work just fine, as would most mods. But if you're changing values in the existing XML entries, then you'll cause all sorts of other issues without any easy way to make sure you didn't break anything.