Greninja is so frail that a SE hit from opposing Greninja is always going to KO
With a naive nature:
252 SpA Life Orb (custom) Grass Knot (20 BP) vs. 0 HP / 0- SpD (custom): 101-120 (35.43 - 42.1%) -- guaranteed 3HKO
4 Atk Life Orb (custom) U-turn vs. 0 HP / 0 Def (custom): 268-320 (94.03 - 112.28%) -- 62.5% chance to OHKO
252 SpA Life Orb (custom) Hydro Pump vs. 0 HP / 0- SpD (custom): 147-173 (51.57 - 60.7%) -- guaranteed 2HKO
252 SpA Life Orb (custom) Ice Beam vs. 0 HP / 0- SpD (custom): 116-136 (40.7 - 47.71%) -- guaranteed 3HKO
252 SpA Life Orb (custom) Hidden Power Fire vs. 0 HP / 0- SpD (custom): 86-101 (30.17 - 35.43%) -- 33.47% chance to 3HKO
252 SpA Life Orb (custom) Dark Pulse vs. 0 HP / 0- SpD (custom): 97-116 (34.03 - 40.7%) -- guaranteed 3HKO
Greninja sets would only carry two things that are SE to an opposing Greninja, a 3HKO and a possible OHKO. U-Turn is the closest thing to an OHKO and that's only a chance. This is of course assuming that the enemy Greninja hasn't changed type yet. If it has, then things get more complicated. So not only do you have to win the speed tie, but you have to hit them with U-turn and switch out hoping that it KOs them and they don't hit your switch-in for SE damage (Which is possible considering his coverage).
You have one move with the
potential to OHKO an opposing Greninja, and it also forces you to switch out, that is IF you win the speed tie. Why take the chance? If they go before you and change type to bug, water, or ice then you're going to do:
4 Atk Life Orb (custom) U-turn vs. 0 HP / 0 Def (custom): 90-107 (31.57 - 37.54%) -- 88.38% chance to 3HKO
and god forbid they use HP Fire on the prediction:
4 Atk Life Orb (custom) U-turn vs. 0 HP / 0 Def (custom): 44-53 (15.43 - 18.59%) -- possible 6HKO
The mind games tie in a lot to the 50% chance on the speed tie. Your only move to defeat a Greninja is only a 31.25% (50% speed tie, 62.5% chance to OHKO) chance to do so, and it forces you to switch out. Why not switch to an appropriate counter instead of taking the gamble?
I think that if your goal is beating frail Psychics, then U-Turn isn't the best option. Greninja's Attack is still mediocre, no good player is switching in Espeon or Latias, and KOing something with U-Turn isn't a great especially idea, because it gives the opponent a better match-up. Even though a Hasty Nature is what is recommended, U-Turn is still mostly for momentum, with more damage being a plus.
Correct, but a player might switch in a Celebi. The extra EVs in attack aren't really needed anywhere else and will give you a few nice KOs on Psychics and Darks without running Dark Pulse. It doesn't really matter if you gave your opponent a better match up because you just killed one of his pokemon. All you have to do is switch pokemon again, via volturn or what have you.