@syrim, what's this "online" thing that you were trying to hack in your second tab? :p
If OHKO moves *were* competitively viable, I would be in favor of NOT banning them. If Sheer Cold Kyogre was a reliable check to Sleep Talk Giratina or CM Arceus Steel, then I'd feel that there was some good that outweighed the stupid.
But as others have pointed out, phazing is much more of an effective strategy (in the case of Sleep Talk Giratina, note that the odds of you pulling off Sheer Cold are LESS than the odds that Giratina selects Dragon Tail), so the only time OHKO moves are going to be more "viable" is when your opponent is down to their last Pokemon, in which case you're winning or losing a match on hax.