Just realized I've already had this conversation before. Whoops.
Anyways, because Whimsy can learn Worry Seed, when used in doubles with Prankster, Worry Seed, and Regigigas, let's it become a rape train, basically. It turns Gigi into a pseudo-Uber. 160/80/100 attack stats, 110 all around defenses. Pair it up with, say, special 'Mence and it's basically gg.