@Mr.Prince:
…....He'd take Usopp's place as "Long Nose-kun". Robin will have to result to Long Nose-kun 1 and 2. At the start, I thought Kaku would be the new crewmember of the Mugiwara crew. As the story progressed into the Nies Lobby arc, bam! Kaku was a CP9 guy and my thoughts were shattered. Mr. Oda gets me everytime.
Or maybe Usopp would be "Sniper-kun" and Kaku would be "Giraffe-kun".
And you can't accuse Vivi of being a deserter. For one thing, if she had become a Mugiwara, she'd be putting Alabasta on the line. Second, she has a family. A father (real father) and Igaram and Teracotta. Plus, I think she'll end up marrying Kohza by the end of the series.
The marines would have had no problems taking her family hostage if she had stayed with the Mugiwara. Plus, she has a kingdom to lead.
All of the other strawhats have no obligation to anyone but themselves. Nami's village is free, Luffy and Zoro are drifters, The Ishii-20 became good again and Wapol was defeated, Zeff has plenty of other cooks, Galley-La has plenty of Shipwrights, Robin has NO ONE, Usopp wants to become a brave warrior the sea and has no reason to stay in Syrup Village (Kaya's safe now). Vivi had a lot of responsbility and she couldn't just abandon that.