Stat Changers and Effect Inducers are important. They bring variety and spice to a moveset. I mean how interesting is a Pokemon with Ember, Flamethrower, Fire Blast AND Fireball. Haze especially should stay for flavour reasons. It fits Koffing more than practically anything on its moveset. There's nothing wrong with just juggling the levels around a bit and just adding to the moveset. That's what Gamefreak does from game to game anyhow.