In answer to the original post, and topic.
It was sometime in the mid-late 80s. I had grown dissatisfied with the worlds being produced, and after Forgotten Realms and the 1st Ed Oriental adventures came out I saw how you could use your own world, with the mechanics provided. Up until this time I had only ever played TSR games, D&D and Gamma World pretty much.
I discovered Champions at University in '85. A few years later, I had Champions, Danger International, Justice Inc, and others - same game system different settings. I started crossing them.
Then came 4th Ed Champions. System here. Setting there - completely independent of each other. At that point I saw the fact that the two could be decoupled and you can tweak and massage the rules to support the campaign feel you wanted in your game.
I trusted the game designer to make balanced rules I could use with game.... but I didn't trust them to be able to give me the gameworld I had in my head. And that is the big point - I have never used a published setting in my life, every game I have ever run has been a homebrew. So, no, I ignore Gary's tirade against guns, or Zeb's comments about magic items if they don't fit my world.
It's not a lack of trust, but the fact they aren't in my brain, knowing what kind of game I want to run.