Tbh, I have never thought of Phione as a Legendary. I always assumed Phione was like a Pokemon where you can only get it at one place and then afterwards be able to breed it, for example like Spiritomb or Larvesta. xD My opinion is Phione should not be considered Legendary because it can breed and so it isn't rare like a Legendary should be.
Personally, because it can breed from a Manaphy, which is a Legendary Pokemon, it should be special whether or not its breedable. I mean, I don't see any complaints about [email protected]
Since Phione is an offspring of Manaphy, should Phione know Heart Swap as well?