Exactly my idea as well. Even Luffy needs to lose every once in a while. And don't give me that Aokiji beat him story. Yes he beat him to almost death but he wasn't a villain that wanted to fight.
I would like to see Luffy get defeated by someone that is the arc villain. I still believe Luffy beating Lucci, and the Going Merry saving them was just Oda being lazy and using a deus ex way of making the heroes always win.
My main point is that the Straw Hats need to lose so they can get stronger.
Robin leaving them and agreeing to go with CP9 on Water 7 was a defeat to them. When they found out she was doing it to save them it made them stronger.