Something tells me people are forgetting that SwagKey also often includes Substitute and Thunder Wave along with Foul Play. Think about it. Substitute means that if they attack/snap out of confusion, it won't do nearly as much damage as it would. Thunder Wave is rather obvious, leaving an even smaller chance of actually attacking and also pretty much guarantees that Klefki will outspeed them even with its subpar speed. Foul Play, well, runs off of the opponent's attack stat which makes Swagger all the more helpful.
[/rant]