Spirit Shuckle is good because you can trap monsters, while dealing damage. If you are behind a Sub you can easily trap monsters you are supposed to beat 1 vs 1. The problem is: what monsters this thing can actually beat?Spirit Shackle / BP / (Feather Dance/Synthesis) / (SD/NP) looks super toxic to me. Y'all laughing at it but guess who you'll call prophet when it gets the boot from OU.
Is this enough to justify a spot into OU? In my opinion, no.
I think that Decidueye can beat only slow, fat monsters with the combination of Spirit Shuckle to trap things, Substitute to avoid status, Toxic (not even Leeech Seed or WoW) + a filler move.
Too bad Game Freak wasted so many points into that SpA stat (Spirit Shuckle is physical) instead of its Spe.