At those cost, they can have powerful effects but still be fair, they can win the game if left unchecked but be vulnerable to spells and/or creatures, they can protect themselves from one but not the other, they can generate card advantage repeatedly, etc
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Using these spells early can remove the counters before they ever become a factor
It often enables ninjutsu or sneak, its a cheap attacker for Winota, Joiner of Forces, and its an artifact creature with battle cry for Graaz, Unstoppable Juggernaut
Its removal in Green with some staying power because of the extra abilities of reach and Ward 2
Rule Zero Its worth noting that some Magic players arent fans of playing against mill decks