Pyroclasm's 2 damage sweeps away Goblin Lackey, Goblin Matron, Goblin Piledriver, Mogg Fanatic, and the Siege-Gang Commander tokens, all of which have toughness 1 or 2, making it excellent against this low-toughness aggro swarm. Armageddon is still strong since it can strand goblins that need Rishadan Port or Wasteland mana, and Swords to Plowshares answers Siege-Gang Commander or a buffed Piledriver cleanly. Call of the Herd comes out because its 3/3 tokens are too slow to stabilize against a fast aggressive curve, and Pyroclasm is a better rate against their swarm of small bodies.