@RazgrizX:
I don't think it would be that unbalanced, it's not like this is math and 2 Shichibukais > 1 Shichibukai or DB in which power levels is all that matters in battles.
It wouldn't be that huge a difference overall, since Jinbe is around Zoro and Sanji's strength level… but there's still some audience feeling that he's more powerful, and has more experience in the new world. The fact that the complaint keeps coming up shows that. By giving him a little more space and more time for the SHs to shine on their own, (and for their bounties to raise) it takes away any doubt or pressure that Jinbe would overshadow them.
(Similarly, he WAS overpowered compared to everyone else pre-timeskip. But obviously the jump alleviated a lot of that.)
@RazgrizX:
Now that you've said that…Why DIDN'T Luffy at least offer to help Jinbei anyway when he said he had things to care of?
Because Jinbe, understanding Luffy's personality, said it in a way to keep Luffy from jumping in. Because Jinbe feels he has to take care of his burdens and debts alone.
"I've got this one easy thing to take care of, then I'll join you." which Luffy is okay with. The guy has responsibilities because of the whole fishman thing, luffy doesn't want to be a hero, so it'll sort itself out. And despite the fact Luffy has in the past pressured guys that said they didn't want to leave but deep down really do (Sanji, Chopper, Franky) he'd never force a condition on someone. If he has buisness to do first… he has buisness to do first.
And he promised to meet up with him again, so... that's good enough.
If Jinbe had said "I'm going to go talk to Big Mom, then I'll join you" Luffy would have been riled up. Or, if Jinbe had said "I've got a difficult task ahead, if I survive it, then..." the way Nami, Vivi, Robin or Brook did... Luffy wouldn't have stopped till it was dealt with.
Instead, far as he knows, Jinbe basically promised to join up with him in a couple weeks after going off to do some errands for his old crew. (And loyalty to your friends is SUPER important to Luffy.) If he'd refused outright or hinted at bigger problems, Luffy would have made a bigger fuss about it and wouldn't have dropped it so easily.