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You can't hit man lands with it and most of the dorks you kill with it have either already got value from a come into play effect or cost less than the Decay making it not great as creature kill
The mechanic only gets stronger with time as Wizards prints more and more legendary creatures
However, that's a description of the simplest case
Conversely, midrange and control decks feature fewer low-cost spells and a greater number of cards in the 3 to 4 mana range
Definitely a powerful ability, and one thats worth supporting with cards that grant or emulate haste like Lightning Greaves, Thousand-Year Elixir, or Swiftfoot Boots