Because Greninja gets STAB on every move, he's better off remaining flexible so he has as wide coverage as possible. Compare him to Specs Kingdra (who I was really enjoying this last month or so): Kingdra get Ice Beam and it's extremely useful for hitting SpDef Celebi who otherwise walls you fairly well, but even then the power is just lacking a little bit, you need either hazards or previous damage for a guaranteed 2HKO, but if she switches in on Draco or Hydro Pump when fresh you're forced out. Greninja can use something like Spikes or Hydro Pump Celebi switches in and then immediately force it to retreat with a STAB Ice Beam which does 60% minimum. Also kinda what flUddOS said. Choice Greninja isn't bad but keeping him as flexible as possible will make sure he does the most amount of work possible. If you played the No Stealth Rock ladder, think of it in terms of DD Dragonite. No, mixed attacker Dragonite isn't bad but DD Dragonite is so fucking good without Stealth Rock that you're losing games you should otherwise be winning.
CarnivorouSwarm: I'd say it really comes down to his lackluster physical movepool. 95 is perfectly serviceable (Kingdra makes it work, though he typically runs a boosting nature as well) and physical Greninja isn't bad by any stretch of the imagination, but he's like top UU or bottom OU as opposed to top OU, if that makes any sense. Why have the Hershey's bar when the gourmet French Silk pie is also right there? (no you cannot have both.gif)