A Pokmon meant to cause Sleep or Confusion is optional, but Id go so far as to say Vileplume is required, especially when its both a raised Retreat Cost and multiple Special Conditions
Cards which are simply wrong for the format (Balance, Worldfire) obviously wont get the same kind of consideration as more gray-area cards, like Painters Servant or Protean Hulk
This organization makes it easier to locate specific cards, facilitating smoother transactions and trades
It has a good chance to get silly, in which case it could even see the price go up rather than simply hold
Legendary Pokmon cards like Charizard or Blastoise can fade when exposed to UV rays, which lowers both their aesthetic appeal and Pokmon card value
all save Doomsday are legal in Modern