Rotom? He could be considered legendary. You only get one throughout the game. But to stay on topic, Phione. It's so much weaker than Manaphy, plus you can only get it through breeding, so what's up with that? I also hate Manaphy, LEGENDARIES WERE NEVER SUPPOSED TO BE BREEDABLE IN THE 1st PLACE!! I guess that kinda kills Phione's "legendary" status... :P
Sorry for this off-topic stuff. But Cresselia DOES have a point to it. If you played through the whole D&P game(s), You'd know there was a boy trapped in Darkrai's nightmares, causing you to go to an island to find cresselia. Once you get to it, It would fly away and drop the cure to the boy's nightmare. I don't remember if there was a reward, but whatever.