Huh, that is a little interesting. At a glance, I'd guess that it's probably because Cresselia can use Wish, then Ally Switch so that its partner gets the Wish healing. That way, the damage from taking a resisted hit is negated, plus you get to mess with your opponent.
Except that Wish isn't even listed as one of Cresselia's top moves?? I guess Cress is just tanky enough to make it work. The reason I'm saying that Ally Switch isn't justified is because I'd imagine if your opponent gets even one prediction and outplays you, they can take out or at least severely damage both of your Pokémon, while a good play from Ally Switch will rarely allow you to sweep the field (although Cress being immune to EQ remedies that and is probably a factor for its high usage; on other Pokémon, this would be a big problem). It just seems like a big risk for only an okay reward.