I think you are correct in saying that Decidueye is not meant to be a brute force monster. However, i don't think he makes that great of a utility mon either. Whenever i use his defensive defog sets or stallbreaker sets, I always feel disappointed because they either take away your offensive momentum for little reward (defensive defog) or they still get beat by whatever you're trying to eliminate if they run the right coverage or support move (stallbreaker sets). The issue really stems back to stat spread: he's not bulky enough to outlast most switchins to him, even with roost (which also saps too much momentum) and he's not scary enough to force most walls to respect him. I almost gave up on him after playing this way, but in the last few matches, i think I found a really good niche for him in the OU metagame . . .
Decidueye @ Assault Vest
Ability: Overgrow (Long Reach)
EVs: 32HP / 252 Atk / 224 SpD
Adamant Nature
- Sucker Punch
- Low Sweep
- Spirit Shackle
- U-turn
I believe he works best as a specially defensive pivot that can actually threaten switchins. He works especially well with bulkier teams that run an Intimidate user (i.e. gyarados, landorus-t, etc.). The idea is to sponge hits for your team from offensive electric types, water types, fighting types, and weak coverage moves. For this reason, he is EV'd to be as specially bulky as possible (though i am sure someone can come up with a better, more efficient spread). Your most common switch-ins are usually going to be steel, flying, grass, and dark types. Low kick is very important to this set, not because of coverage, but because of the utility it provides while dealing damage. Slowing down an enemy before switching out to a check or counter can be a godsend for certain teams that focus more on bulky offense and do not want to take many hits before retaliating. It can also force your opponent to play mindgames with you, since your priority will now outspeed most mons.
I was thinking along this line myself. Its ace in the hole is mainly its godly movepool. I just wish that it had a draining offensive move like Horn Leech (or Leech Seed)