Imo oda had to give bb a real match up with the other pk candidates. Luffy has zoro, buggy has mihawk and shanks has Ben Beckman. Before bb fit the standard of the other yonko which shyriuu filling that yc1 role. But with the rave of pk falling down that model wasn't enough and bb needed another to tier to compete with the current model of yonko crews. Imo it's a good way to flesh out kuzan and once we get to bb we can get a indepth look at the flaws of the marines through kuzan if oda wants. Imo having kuzan join gave narrative purpose to kuzan and built up interest in bb not only in world but in real life. If kuzan didn't join he wouldn't have any narrative and he's to strong a character to just waste doing nothing being written out of the story. He on par with akainu and was a candidate for fleet admiral, no way he should have just been a wondering drunk doing nothing.