I will make three points to back up my statement.
1. Jinbe has nowhere to go in Fishman Island
2. Jinbe has shown that he can fight alongside The other strawhats.
3. Jinbe has connections to people's past (Arlong)
Jinbe is basically an outlaw, The only place he can go is the Merman District but he won't be satisfied staying there. Jinbe is most likely not going to stay on Fishman Island because its going to be destroyed(prediction). Jinbe doesn't seem like the person who is okay staying in a place without human interaction.( though you could make the point he spent two years doing nothing that we know of).
Jinbe could be very valuable to the strawhats, seeing how he has vast experience about the Grand line. His strenght does not seem to vastly outpower the captain (luffy). He will most likely become the Helmsman. Most importantly he adds more diversity to an already diverse crew.