I think there is a general preference for mono-type movesets on attack, since you'll more likely to exploit some type of type advantage. To the best of my knowledge, there is not a mainstream defender in the game that is weak to both dark and rock. However, there exists some defenders that you can use a double dark much better (e.g. Exeggutor, espeon)
But in this meta I think B/SE is more useful. Hits gyarados, dragonite effectively. Rather have that than exeggutor or espeon.
Then again, whenever the legendaries come out, the cats in particular, then the B/C godzilla is by far the best counter to them, unless the cats have counter and focus blast then game over for godzilla