Most of the needed positions on ship are covered (especially the Musician part yo ho ho ho) so if the SH's are adding someone it should be to further the image of Luffy being a destined pirate king and playing a pivotal part in the final war. Jinbei joining to represent the Fishmen fits in perfectly, along with adding an expert in Fishman Karate.
Bartlomeo looks like he belong to the same race as Moria and maybe even Kaidou, plus he's get an awesome defensive fruit that will serve well to protect the ship. Plus he can now replace Sanjis place as a nosebleeding, crazed fanboy. What does Rebbecca add? Whe already have a (real) Kenshi.. Please don't say boobs, that's cheating :(