User blog comment:WillofOP/Almost Nakama/@comment-3569609-20111208064355

Jinbe is a tank, he can ignore pain quite easely as shown in ID and Marineford, he is also amazingly strong, he can swim extremely well and he can steer a ship, so i would say he is the smart and serious fighting force, i think he could also be the one steering the wheel.

Anyways, at this moment Jinbe is obviously as strong or probably even stronger then Luffy, so it makes sense that he isn't joining now while he is stronger, but later when he is weaker then Sanji or Zoro because I think Oda wants to keep the monster trio.