While I feel that Ditto is not a top tier OU threat, it successfully shuts down some of the most dangerous set-up sweepers in the tier. For example, the much hyped DD Outrage Moxie Mence is completely shut down if they opponent has a Ditto, as the player with Mence will not like the prospect of being counter swept by their own Pokemon. Ditto cannot be thrown onto any team, but if you look at your team and realize you're weak to set-up mons, then Ditto is the right man for the job.