Focus Sash isn't necessary when Weavile aims to punish only non-STAB U-turners(Landorus-T) without boosting items:
252 Atk Landorus-Therian U-turn vs. 4 HP / 0 Def Weavile: 236-278 (83.6 - 98.5%) -- guaranteed 2HKO.
Attacks from Life Orb Rillaboom or Scizor is more likely to trigger Focus Sash...