"If". Unfortunately assumptions like that don't matter much. If the metagame was oriented towards a more defensive gameplay, we wouldn't be discussing this right now.
The fact is that Poison, Grass, Ice, Flying, Rock and Bug are "mediocre" types if compared with Ground, Dragon, Water, Steel, Ghost and maybe Dark. The usable Pokémon of the aforementioned "mediocre" types usually are paired with another non-mediocre type, like: Poison/Dark (Drapion), Ghost/Poison (Gengar), Grass/Steel (Ferrothorn), Flying/Ground (Gliscor, Landorus), Flying/Dragon (Dragonite, Salamence) Rock/Flying (Aerodactyl, Archeops) and Bug/Steel (Scizor, Durant, Forretress, Escavalier, Genesect), Bug/Fire (Volcarona).