In non-competitive situations where players are just enjoying playing the game for fun and not in any sponsored or official setting, any rules on what cards can be used would ideally be agreed upon by those involved
Elder Deep-Fiend Elder Deep-Fiend has an ability that taps down permanents at instant speed thats very useful, especially when you want to clear the way for a beefy sea creature
The drawback is that Zekrom ex does 30 damage to itself, but with 230 HP it can usually handle that recoil without too much worry
Copying spells already isnt that powerful because lining the copy card up with a card worth copying is quite difficult to do
The available characters are diverse and varied in terms of both heritage and gameplay, with classes like Guardians and Mystics, for example, which clearly indicate what players can expect from them
It has the added benefit of being able to grab a Supporter or a VS Seeker in a pinch, but its primary use is to help a deck pull off difficult card combinations