> implying I use Ditto to revenge kill
Ditto is probably the best scout in the game. What could be better than instantly knowing all of your opponent's attacks?
I switched him into a Starmie once and saw Thunder Wave, Thunderbolt, Ice Beam, and Rapid Spin. As soon as I saw it I learned that Mamoswine was a free switch-in every single time. Starmie couldn't touch him.
I switched him into Terrakion and saw Quick Attack. That told me he was Banded and I was able to react accordingly.
And not only that, if you have the balls to speed tie, win it, and KO, there's a good chance you can get a second KO because everyone will switch in something that resists whatever move you used to KO with because _everyone_ will think he's scarfed.
Of course, this can backfire too: I switched into Volcarona at +1/1/1, saw Hidden Power and went for the Fire Blast. He won the speed tie and I learned the hard way that he was using HP Rock. X[
The biggest problem is that Ditto has absolutely no synergy with any team. The best you can do is use him to revenge kill, but he fails if the opponent doesn't have a move that hits themselves super effectively.