Every pokemon has their niche. If you need a check to Ferrothorn and Gastrodon that can maintain momentum, etc. then Rotom-C is useful. If you have a team weak to Volcarona of course you won't use Rotom-C, but this weakness doesn't make Rotom-C useless. Just because a pokemon is weak to certain types doesn't make it unusable. Just look at Scizor, it has a 4x weakness to fire and can't beat Volcarona and yet it is consistently one of the top used pokemon. Just because X pokemon is weak to Y pokemon doesn't mean its unusable.