Is curse even used at all right now?
I've always seen it as literal suicide on any pokemon that is a ghost type.
Curse never has really ever been viable in competitive play, but I suppose there is some merit using it against stall teams. Otherwise yes, you are sacrificing 1/2 HP of a decent defensive Pokemon just to get off passive damage against a team that might take more for just a direct hit (especially in an early meta where hyper and bulky offense is king).
On a separate note, I feel like there are so many gimmicky sets on this forum right now in general - perfectly normal for the start of a new generation, but what I'm not seeing is consideration of
consistency - basically asking the question,
how often does this set do what it is supposed to, and how well does it do it? In this instance, Curse is rather inconsistent, as it drops you at a really sub-optimal HP range just for the CHANCE to do some passive damage. You give your opponent a free guaranteed -50% for -25% on their Pokemon at least, but if Decidueye gets KO'd very soon after, you're down one more Pokemon that could have served better use in the battle. The same instance can be applied to moves like Hyper Beam or...Slaking. Sure, you hit hard, but is it worth giving the opponent a free turn? Life Orb and Choice items are different because YOU, the user, are given an IMMEDIATE ADVANTAGE over the opponent in your power, then the cost comes later, and even then, is more easily worked around.
Anyway...I feel like either the meta hasn't settled enough for us to really see Decidueye's use, or it simply isn't good enough for OU. I'm hoping its the former - say what you will about Grass's offensive coverage, but hitting Water/Rock/Ground isn't too shabby given all the Tapu Fini and ground types running around. Unfortunately, the meta is so full of Steel types that Decidueye just gets flat-out walled by a lot unless you're running an obscure coverage set. Regardless, the main reason to ever use Decidueye is
Spirit Shackle. No, it is not as good as Shadow Tag and Arena Trap, nor will it ever be, simply because it takes a turn to get the trapping off. However, Decidueye most likely works best with some mindgame play in there - predict switches, trap something you wall/counter, gain a bunch of momentum. The fact that it can do this with Defog and Roost is great too, which makes Decidueye a early or midgame pivot, like what everyone else is saying.
You can be given some defensive/utility-based wiggle room if you run Tapu Fini or an Aromatherapist/Heal Bell user on your team too, because Dedicueye gets worn down by all hazards and status. I can see it working well with Tapu Fini as a core due to their solid type coverage and balanced offensive/defensive stats. A steel type, like Celesteela, can cover Poison/Flying moves too. You can also run Volt-Turn, as Spirit Shackle just sets up all kinds of momentum-based mindgames and plays, as long as you have your side of the field clear of hazards.
tl;dr - we haven't seen Decidueye's skillcap in the meta yet and may not for a while, but I feel like there's a lot of potential to be determined.
Edit: the posts above me basically summed up my points as well.
