Lots of Stoneforge Mystic decks have emerged in Legacy, mostly as a response to the popular removal spell at the time. Right now, Lightning Bolt (and to a lesser extent, Swords to Plowshares) are incredibly prevalent. Abrupt Decay has mostly fallen by the wayside because the color black took a huge hit once Treasure Cruise was printed.
How can we exploit that?
Echecetmat35 prefers to play things like True-Name Nemesis and Geist of Saint Traft, but those cards are a little on the slow end, especially against a combo deck. In order to make up for that, he's playing not only Deathrite Shaman, but Noble Hierarch!
Sure, those things die to Lightning Bolt, but that means the Delver decks aren't playing a threat on turn 1. Plus, if they have to Brainstorm or Ponder into the Lightning Bolt, it means you probably get a turn to play your powerful three-drop. Additionally, it means your Stoneforge Mystics aren't the only Bolt-able creatures in your deck, which makes you more likely to actually untap with them.