Modders are freeloaders that use DEVELOPERS design and tools to modify a game. They are hardly professionals and most likely do NOT test what they have made many times over or use others to test the BALANCE issues of what they have done. Graphic modders are another thing and I have no qualm with them. But, those that change number around and core things that are possible to change I will always be leary of and so should everyone else.
I would beg to differ. Modders sometimes do often make original stuff - sometimes total conversions are often better than the games that were before them.
Even XML modders can be good - look at some of the GC2 community mods for example. They have through their hard work managed to make the game a lot better, improving balance, the AI, and quite a few things.