As a game designer, I find the notion of unquestioningly trusting every piece of rules that comes from any game designer troubling. If Gary wants to say that in Greyhawk, gunpowder doesn't work, which is why he didn't include guns, that's cool. If the player wants to include it, that's cool too. If Gary says that guns can's work in D&D, that attempts to take choice away from the DM.
I'm all in favor of DMs tinkering with their own games and making decisions about what does and does not appear/work in their own games. Because these are roleplaying games, I consider any rule in the game as optional (keeping in mind that if you decide to change something, you must then deal with the ripple effect throughout the rules of all changed things). For that reason, if someone were to take a prestige class, spell, monster, or some other thing that I wrote and declare it broken, overly complicated/not complicated enough, incompatible with the flavor of your game, then I'm in favor of not including it in their game. My philosophy is that the game mechanics were tailored for you to use, not use, or modify as you see fit. Once I've done the best I can with them and put them out there, it's out of my hands and in yours.