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Rock/steel isn't that bad defensively. I wouldn't call it a good typing because of how common ground/water/fighting is but its not absolute bottom of the barrel. It's better defensively than quite a few other combinations, which is a testament to how OP steel is. When I think of something that's one of the worst defensive typing, its something like rock/ice, rock/ground, fire/rock, bug/flying, bug/grass, etc.
A lot of rock/steel pokemon happen to be bad, but thats for reasons that go far beyond typing.
