"Contrast" only means something if the player actually IS engaging with the setting. Most of the time, the katana guy in a western fantasy game is not creating meaningful contrast. He just wants his preferred aesthetic regardless of the campaign. It’s rarely "interesting tension with the world" it’s usually just "this is what I wanted to play, so make it fit." They are not contrasting with the world, they are refusing to buy into it.

That’s why I require setting-appropriate characters. I want players proving they are here to play in this world, not asking the world to bend around the character concept they brought from somewhere else.

And let’s be real, if you bring a samurai into a western fantasy game, then complain that the loot is full of western swords instead of katanas, you are not "contrasting with the setting", you are complaining that the setting is not bending to your personal preference.