Considering the beauty of Grass/Fire/Water is the equality of the trio; dark is immune to psychic and thus has a relative upper hand.
What makes this especially dumb is that there actually IS another trio that's actually equatable to Grass/Fire/Water: Rock/Flying/Fighting. Each type resists one and is weak to the other. No immunities.
Of course the other aspect of Grass/Fire/Water that people seem to forget (for some reason) is that Grass resists Grass, Fire resists Fire and Water resists Water, which is another thing Dark/Psychic/Fighting misses out on, for Fighting at least. Then again, so does Rock/Flying/Fighting, but at least those three are all neutral against themselves, rather than having another inequality.
In summary, people who advocate Dark/Psychic/Fighting are by and large terrible and they probably smell bad.
King of the Zelda Wii U WT Board http://i.imgur.com/RK6EB.png