@Kamakiri:
You are viewing things from an "in-universe" or "character" level. Although looking at things this way absolutely works in hindsight ("Paulie never wanted to join the Strawhats, which is why he didn't"), I think it is less useful for making predictions for one simple reason: Oda is the author, and because he is in absolute control of all characters and circumstances, he can make whatever changes he needs to get a character where he wants them to be.
Example: Franky was introduced as an enemy whose gang beat up Usopp and stole money from the Strawhats. Then he battled Luffy and was a major douchebag. Midway through the Water 7 Arc he absolutely did not want to join the crew, and there was no way the Strawhats would have had him anyway. At this point in the story, readers making predictions by thinking from the perspective of the characters would never have guessed Franky would join (they might pick someone friendlier instead, like Kaku. Obviously there were some things about Kaku that readers would not know!) Of course, during the CP9 Saga Oda changed circumstances and developed characters in ways that allowed Franky and the Strawhats to reconcile their differences and build trust.
Although at this point only Jinbe and Bartolomeo have shown clear interest in joining the Strawhats, this does not mean that they will join. Just because other characters have not shown interest does not mean they can't join. Just because a character has obligations or difficult circumstances or said something in the past or has a rocky history with the Strawhats does not necessarily mean that they will not join. If Oda wants it to happen, he can make it happen.
…and this is why I think it is more important to try to predict authorial intent than character intent, at least during this sort of discussion.
that's why i said it can change, by the end of this arc some characters might display an interest in becoming a SH. But at this moment only Jinbe showed this interest and Bart, due to him being a fan of this pirate group. Kinemon also showed an interest, well maybe not joining but at least he is interested in strawhats.
I never mentioned any specific circumstances of why a character can't join, i just said they have to have an interest in it. I personally don't see the problem with someone being a royalty, a villain or whatever.
My point was that's it's more meaningful to discuss whether certain character will join or not if he has an interest in joining. Without it, any discussion is pointless, without this aspect any discussion falls into category of personal preferences until he/she shows this interest - I like Rebecca i want her to join, i can say i like fighting fish, it would be great if fighting fish joined - that's all personal preferences until character himself will display some interest in joining