@Nekketsu:
There're a number of reasons with wich you could agree more or less or not. Because of his experience in The New World,
Robin and Brook already had experience on the grand line before they joined.
already and because I just can't picture wich his role in the crew would be.
Because they really needed an archeologist and musician, right? Ship role is entirely secondary to how Luffy feels about the person. Franky is the only time they were actively seeking a particular position. Navigator, cook, and doctor were all things they needed, but Luffy recruited the person first, then they happened to have those talents. Usopp certainley wasn't hired to be a gunner.
Aside from that, Jimbei's most obvious role would be undersea scouting and saving the FOUR DF users in the crew who keep going overboard.
But the main one is because Jinbei is the captain of a pirate crew already
So was Usopp, Franky, and Brook.
and was part of the 8 from Impel Down.
All the better. He's fought side by side with Luffy and seen his qualities. Not to mention helped him tremendously after Ace's death, and the whole "vow to protect you with my life" bit.
Oh, and even though I don't know if Luffy is now equally strong or stronger than him, he's way stronger than Zoro without any doubt, and this… just no.
Underwater sure, but Zoro can cut a boat in half with a single swipe now, and can crush a Pacifista in one shot. (And so can Sanji.) The entire crew is strong now, and they'd be on land most of the time. Jimbei would certainly be in the high tier, but he no longer outclasses everyone.
That was actually a problem pre-timeskip. Its sorted itself out now, along with others.
@Mr.333:
Just a thought on my mind, but we do have all this mysterious, World Government connection to Bonney (who most likely isn't a pirate captain at this point.) Akainu even said that her escape "gave him shivers", so even if she's not present during the Fishman Island Arc, I wouldn't eliminate her from the NN Race just yet, not until we know more information at least.
Bonney isn't joining.
And we already have two girls in the crew, in a world that doesn't have a whole lot of female pirates. Only one female supernova, (1/11) one female yonkou,(1/4) one female shichibukai (1/7), one female BB pirate (1/10).
I know fans want more service, and balancing out the ratio a little would be nice, but there's really no reason to expect another girl to join.
And there's absolutely no way a mermaid under the age of 30 is joining. Camie is showing pretty well that someone needs legs to get around.