It however is still considered to be in exile and not on the battlefield or your hand, meaning that there are much fewer ways for your opponent to be able to get rid of it before you can cast it
While all Spells and Traps go in the Main Deck, only some Monsters are allowed in the Extra Deck
If I were to use him, I would like to use him on Vrynhildr, who can be expected to have a place in Valkyrie and her ATK, but I can't deny the feeling that even if I use it, it will only grow once or twice
Leyline of Transformation I know Magic players whose entire personality is trying to pull off some strange combo with type-changing effects like Maskwood Nexus or Arcane Adaptation
Commander Eriette, the Beguiler Lifelink Whenever an Aura you control becomes attached to a nonland permanent an opponent controls with mana value less than or equal to that Aura's mana value, gain control of that permanent for as long as that Aura is attached to it