Originally Posted by Fat magic9mushroom
This is true in RBY OU solely because the best Psychic-types - the two bulky Psychics with Recover - were the original Uber tier. Moreover, the Normal-type that stops most Psychic-types - Chansey - has utterly no use of its STAB and would be strictly better were it Psychic. For that matter, literally every single-typed Pokemon in RBY would be strictly better were it dual-typed with Psychic. The same is not true of Normal.
Every single-typed Pokémon in RBY would be strictly better were it dual-typed with Ghost, too. And some Pokémon would have more to gain from Normal-typing than from Psychic-typing.
But you can't mix and match your different kinds of arguments. Either you look at the OU metagame as it is--led by three Normals--or you look at types on their own merits--comparing the value of Normal STAB vs. Psychic STAB + resistance vs. Normal immunity vs. Ice STAB + resistance + immunity to freeze, etc. The moment you try to take on both tacks, your argument becomes incoherent.