That's why Gliscor totally wins over Skarm. Plus anyone carrying Magnezone loses a major part of its usefulness. Gliscor can take anything like a man, tbh, even things Skarm can't switch into like a really strong thunderpunch ^^ Your infernape does 32-38% on a max def/hp Gliscor, btw, and Gliscor with 4 attack EVs does 94-111% on 4HP EV Infernape. So it takes it, and dishes it back x4.
Since we're talking about things to use instead of Skarmory, Gliscor has to be the best Roost user out there. A lot of physical Pokés carry a special fire attack to mess up Skarm, too; But how many bring along ice beam? Not too many.
Roost shedding off half of his ice weak is really nice too, but you shouldn't be staying in on an ice attack anyway :P Most Ice users are special, so it's a weakness he doesn't have to worry about unless he's in on something mixed with ice.
Meh, they're a bit like golden apples and granny smiths. Wall vs Tank? I just personally prefer Gliscor.