@Ivotas:
True. But still that doesn´t erase the fact that Robin went through some major character development while the arc was centered around her. So the interesting part is, how much more of a development she has to go through in order to get her number title?
Yes, but the character development was about her realizing that her life was not a crime, and that she had found her nakama. All the other chapters with these titles have involved the member overcoming a personal barrier which has prevented them from truly pursuing their dream, which isn't quite what happened with Robin. She was already sort of trying to read the poneglyphs anyway, so there must be something else.
The First: Actually, Zoro is kind of the exception here, I guess he didn't really have something holding him back to begin with.
The Second: Nami felt obligated to save her village, and was trapped by Arlong into working for him even though he wouldn't ever give her what she wanted. This stopped her from her dream of making maps of the whole world.
The Third: Usopp wants to be a "brave warrior of the sea", but is cowardly. Sogeking gives him courage to fight, and he also comes to terms with Merry dieing and apologizes to the Straw Hats.
The Fourth: Sanji feels obligated to work at Baratie because he ruined Zeff's dream for him, then finally leaves with Luffy to find All Blue.
The Seventh: Franky wants to travel around the world in a ship he built, but A) won't build anymore since his creation had Tom killed and B) Thinks the Franky Family needs him around. He builds a ship for the Straw Hats and then is "forced" into joining them.
The Eighth: Brook wants to meet Laboon again, but is trapped in the Florean Triangle and not strong enough to get his shadow back on his own. The Straw Hats save him and tell him Laboon is still waiting for him.