Camerupt, Weavile and Magneton are one of the 'mons I never used, alhtough I have already seen them several times in the competitive play. Camerupt because it takes 4x damage from water, Weavile because of poor defenses and 4x damage from fighting. Magneton for low movepool and for taking 4x damage from ground. I find in general that using a Pokermon with double weakness is a huge disadvantage, even more if the type it is 2x weak to is commonly used, like water, ground, fire or fighting.
Unless the Pokemon has a way to cover his disadvantage, like Heracross with Bulk Up, Whiscash with Amnesia, or Gyarados with Earthquake, I really think that it is not worth to use a Pokemon with double weakness in competitive play.
Trainers, share your thoughts about the double weakness: do you like or avoid to use them?