Azorius Tempo fills the board with small efficient fliers (Sage of the Skies, Steamcore Scholar, Momo) that Slagstorm's 3 damage sweeps away cleanly, while Impractical Joke only trades one-for-one with a single creature and can't kill Quantum Riddler's 6 toughness. Spell Pierce is good tempo disruption against their cheap noncreature spells like Daydream, Practiced Offense, Erode and Get Lost, ideally cast while they are tapped low. It can't hit creature spells such as Quantum Riddler, so don't count on it for those. Get Out comes out because its bounce mode only returns creatures and enchantments we own, which does nothing against their board. That leaves only the counter-creature-spell mode, and we don't need the extra copies given the sweeper plan.