Now that's just lazy writing.
Look at it this way: Gym Leaders are the high school coaches, mentoring new trainers directly, while the Elite 4 are the professional athletes that inspired the new trainers to become trainers in the first place. Both serve as role models, but mentor figures are held to a higher standard.Also, aren't the Elite 4 supposed to also be everything the Gym Leaders are? I fail to see how this whole "can be MULTIPLE Elite 4 members but never a Gym Leader" thing works.
That's my interpretation of it, anyway.
I always figured that Fighting was the good/noble to Dark's evil/underhanded. It fits well with Fighting being super-effective against and resistant to Dark.
The idea of "Light type" Pokemon is a pet peeve of mine. It doesn't really add anything to the game.
That actually sounds like a decent gym gimmick. It's certainly better than "Look at me, I'm a Gym Leader who uses Dark-types! Isn't that so cool and edgy!" Which is what most fan-made Dark Gyms seem to be.
The only fan-made Dark Gym Leader that I could take seriously was a crotchety old man with a gambling problem. It made sense in context (he ran the Game Corner), and it was a refreshing change of pace from all the cool young rebels.