But then you miss out on valuable damage against the blobs. U-turn is a great move on Greninja, and provides nearly the same coverage as Dark Pulse.
How does Dark Pulse help against Blissey and Chansey though? 88 Atk U-turn deals more damage to both Chansey and Blissey than 252 SAtk Dark Pulse.
88 Atk Life Orb (custom) U-turn vs. 252 HP / 252+ Def Eviolite Chansey: 136-161 (19.31 - 22.86%) -- possible 5HKO
252 SpA Life Orb (custom) Dark Pulse vs. 252 HP / 0 SpD Eviolite Chansey: 94-110 (13.35 - 15.62%) -- possible 7HKO
88 Atk Life Orb (custom) U-turn vs. 4 HP / 252 Def Blissey: 204-242 (31.28 - 37.11%) -- guaranteed 4HKO
252 SpA Life Orb (custom) Dark Pulse vs. 4 HP / 252+ SpD Blissey: 86-101 (13.19 - 15.49%) -- 9HKO at best
Again, nearly the same coverage as Dark Pulse, hits the blobs for more damage, AND provides momentum.
Let's be fair though, U-turn is mostly about the momentum, no one really cares about whether you're 5HKOing or 7HKOing the blobs when they can just Soft Boil off the damage.