While not every deck runs it, many top tier ones do, leaving quite a big hole especially for decks that rely on having energies in the hand to accelerate multiple in a turn, like the powerful Mega Charizard X ex
And even if you claimed that youre playing it fairly whats fair about a turn-1 4-drop
Necropotence (T1, Ext, S, Block) The Skull
This is optional, and if you have more than one opponentooh, scary!you choose one and only one of them at most
A second example is [c]Sneak Attack[/c]
A heuristic I have found useful for identifying cards fundamental to the functioning of a deck is this: does your deck mulligan a high percentage of hands that do not contain this card or this class of cards