my opinion about jinbe joining is 70/30, reason why?
Positive:
1. he has a very good unique fighting style
2. he has a character development
3. he is unique among fishmen becaise he can talk to fish and in the sense that him joining will give the straw hat a great advantage
4. he already had a bounty that will add up to the crew's total bounty.
5. he has a unique quest for his dream that the is reliable for him joining, uniting fishmen and humans
6. he was shown with a unique fighting stand with the crew like how franky does
7. he is clearly asking help from luffy like how every other member of his crew does before joining
Negative
1. he has a great role of protecting FI.
2. he was never personally asked by luffy to join in which he have to decline then accept.
3. there is no clue of him joining like how luffy predicts franky's looks waited so much for a musician and importance of getting someone in that position like chopper for doctor nami for navigator and sanji for cook.(though Robin is a complete exception of this rule which can also be applied to jinbe)
don't get angry on what i just said its just my opinion. so what do you think?