If rocks didn't oppress entire types, mons like Volcarona wouldn't need them.
>B-b-b-b-but Volcarona is too good without rocks!
There are already mons who are too good WITH rocks in the game, like Landorus and Garchomp for example. The only difference is how bad one centralizing move hurts them on the switch. Now, I think rocks are necessary, because flying mons switching in for free would be too good, but giving up an item for protection is fair.