The reason of this is because Eiichiro Oda said the newest member is an old enemy and Ceasar clown is a great help to the straw hats....think about it what do the straw hats don't have, they've got a chef, a navigator, a sniper, two swordsmen, a shipwright, a musician, and an archeologist, they don't have a scientist and all of the straw hats have a goal to accomplish for example Luffy wants to become pirate king Zoro want to be the world's best swordsmen Nami wants to draw a map of the whole world and the others have one too Ceasars one is to become the greatest scientist...and they more...there is a lot of things Ceasar can do for example he can upgrade the ship so it doesn't need cola and he can upgrade the Clima-tact and other so these are my reasons for Ceasar to be the newest member I know a lot of people hate but people also disliked robin for being bad so it's kinda the same