User blog comment:Neowitch/668 - Pet Axolotl/@comment-3119203-20120523162512/@comment-4605926-20120525150634

I've just got a couple of points to add on this one - Luffy does not shrink anymore after using Gear Third which is going to make it far more useful than it was before. He also learned that Hardening technique (of which I don't know the origin).

Also, Luffy can now start Gear Second much faster and more easily than before as well as turn it off very quickly the moment he no longer needs it, which very likely saves alot of his stamina during a fight.

Therefore, I disagree with your point on Luffy. Zoro and Sanji also hadn't shown any particularly new techniques either (just like Luffy, they both only upgraded their already existing moves) aside from Sanji's Geppo and Blue Walk.

Lastly, we don't know yet if Zoro and Sanji possess both of the normal Haki types, and even if they did, learning one more could be a huge difference, and I doubt that Conqueror's Haki would be completely useless against a strong opponent.