Because he hits 13 out of 17 types for super-effective damage, and hits everything in OU except Cresselia, Rotom, Swampert (if lacking HP Grass), and a few others for super-effective damage. And because of the fact that he's uncommon in OU means teams might not be prepared to directly counter/wall him, which can make a potential sweep, or server dent in the opposing team. I don't care how many times you people put Elecitvire and myself down for trying to get him respect, because all you're doing is letting MixVire become even more of a surprise and sweep teams.