Nobody competent truly believes this, but a Pokémon with a double weakness should have some very good redeeming qualities to compensate for this obvious vulnerability. Particularly for walls this is an issue, sweepers (eg Garchomp, Salamence) are generally less bothered by their weaknesses because they can hit first.
Ferrothorn has a lot of resistances that make up for its single double weakness, for example, as well as a useful movepool so it's not a bad Pokémon. Scizor has a double weakness and is slow, but it has good bulk and an ability that gives it one of the strongest priority moves in the game. Talonflame in the previous gen was in a similar niche (although it's less bulky, strong priority was what made it shine).
And so on. If two Pokémon can do the same job equally well, but one takes quadruple damage from Earthquake and the other does not, it's easy to choose, no?
