Pokémon X and Pokémon Y

I think the reasoning some people have against that "triangle" is that psychic can't do actual damage to a dark type pokemon, whereas fire types can still do damage against water types (even if it's minuscule).

Well, when you first receive your starter, they always have a normal type attack at lv5, you could just use that against the rival's starter instead of using 'not very effective' attacks.