You forget that Tyranitar is 4x weak to fighting and 2x weak to steel, and remains one of the core OU Pokemon. Heatran and Magnezone have 4x weaknesses to the popular Earthquake, learned by 199 Pokemon through TM26. Ninjask has a niche even with 4x weakness to rock and terrible stats. Ho-Oh is Uber even though it takes 50% damage from Stealth Rock. All those Water/Ground types with their 4x weakness to grass? Gyarados is 4x weak to electric, and thus volt turn. Forretress, Scizor, and Ferrothorn are all extraordinarily weak to fire. Salamence, Dragonite, and Rayquaza all have a 4x weakness to ice, and nobody would ever let those three into UU. Typing isn't the only thing that makes or breaks a Pokemon. Besides, that could actually be a killer offensive Pokemon, given enough speed. Those dual STABs are super effective against Fire, Grass, Ice, Ground, Flying, Bug, and Dragon type. They're only resisted by Steel, and basically everything gets Earthquake. (Especially heavy rock type Pokemon.) Every fossil Pokemon after Omanyte/Kabutops has gotten that. Those three types combined leave it resisted by... Bronzong. I can easily see how a hyper-offensive rock/ice Pokemon could be useful. It could be useless, but it's not impossible for it to do well.