Sneak and Show wants to cheat Atraxa, Griselbrand, or Emrakul into play via Show and Tell or Sneak Attack, so answering those threats before they attack or sacrifice away matters less than disrupting their enablers. Force of Vigor can be cast for free by exiling a green card to blow up Sneak Attack or Omniscience the turn they try to go off, buying crucial time, and Haywire Mite offers a cheap backup answer to the same permanents. Pithing Needle only stops activated abilities, so it can shut off a Sneak Attack but does nothing against Show and Tell, which is a sorcery spell. That makes it a narrow answer here, and the extra interaction is more valuable than Pithing Needle, Nihil Spellbomb, or Skateboard, which do little against this combo-oriented plan. Keep Thoughtseize in as your best proactive disruption to strip their Show and Tell or Sneak Attack before they can deploy it.