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.
Because Electivire (mix or not) is a potent threat that has let me sweep many teams unprepared for it. Just like, everyone expects a team to have a lead that cripples SOMETHING, while someone may have a Swords Dance Lucario lead or something. It's not what you'd think, but it is effective if you aren't prepared, which most teams I've rated, in fact, are weak to MixVires. See what I'm gettin' at?
sorry, but I deleted your post in the Drought thread...you revealed part of my team! =P I know it probably doesn't seem like a big deal, but technically you're not supposed to reveal parts of people's teams in tourney threads.