The whole argument about what "role" Jimbei would be assuming he does join is kinda pointless because we already have people in the crew whose role on the ship is kinda pointless, but they're still there.
Namely, Robin. Shes an archeologist. And an awesome archeologist. And thats all she needed. Also Brook was a musician. Much wanted by Luffy, sure, but a musician is still kinda pointless, and we still have one. And he fits right in, regardless.
I personally don't think Jimbei is going to join, but it would be no biggie if he does. I'm not gonna freak is he doesn't have an important or needed role on the ship.