I think this could be one of the strongest reasons for Jimbe not to join, unless his promise to Ace is that reason. I would love to see hime join the crew though, I think if he did, it wouldn't be till they've been in the New World for a decent amount of time and have added at least one more member before hand. But I do think there are reasons for him to join, as far as crew dynamics. I don't remember who said this in an earlier post, but it was mentioned that once Jimbe joins the crew they would no longer be considered rookies. I do agree with this, but counter this point with the fact that they can't be rookies forever. Jimbe adds several quality aspects to the crew. First of all, I think the addition of a fishman would be a filled need. Though they can do this without it being Jimbe, (spoiler) he has already been asked in the manga. Even though he did turn them down, it was kind of suggested that he might catch up to them later. Secondly, where is the true hand to hand specialist of the crew? Yeah, there are characters who can fight without a weapon, but Luffy is a rubberman and everything he does is based on his devil fruit. Sanji is primarily the cook, and, as bad ass as he is, his fighting style is based on kicks. Usopp is the sniper. Franky is a cyborg who is full of gadgets... etc. Thirdly, I don't think Jimbe's nature will inhibit Luffy's ability to be a goof ball and go off on half cocked quest to get the next dinosaure he sees to join the crew. And finally, to go back to something i stated earlier, Jimbe would add a whole other level of credibility to the crew and they could use an older character to even the crew out a little more. In my opinion, in order for Luffy and a bunch of the others to truely achieve thier goals, they are going to have to grow up to a certain extent. But, then again, he could just be that character that is always around at random times but never truely joins as a full blown member.