In playthroughs, you usually want Pokemon that can get you through the game relatively quickly. If you don't care about time, you can just raise 6 random Pokemon to level 100. Thus, breeding for any purpose (including but not limited to hidden abilities) is almost never worth your time.
In gen 7 Monotype grass teams, you'd want long reach.
Long Reach prevents Decidueye from taking contact damage from Pokemon such as Ferrothorn and Garchomp when using Shadow Sneak.
In ghost teams, it doesn't matter which ability you use because Decidueye has no room for any grass moves or contact moves.
Spirit Shackle is Decidueye's main attacking move, used to trap defensive Pokemon like Mega Venusaur, Sap Sipper Azumarill, and Ferrothorn. Roost is used to give Decidueye a form of consistent recovery. Defog allows Decidueye to fill the role of an entry hazard remover, which is helpful for teammates that are weak to Stealth Rock, such as Alolan Marowak and Blacephalon. Swords Dance augments Decidueye's offensive presence, allowing it to muscle past defensive walls like Clefable, Ferrothorn, and Mega Venusaur.
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In gen 7 NU, you'd want overgrow for leaf storm sets and long reach for non-leaf storm sets. The metagame has several rocky helmet users, such as Xatu and Garbodor, that can switch into Decidueye's U-turn or grass knot.
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