User blog comment:The Koromo/Chapter 758 Review: Again/@comment-4493594-20140828204202/@comment-4493594-20140829122732

Fair points, Zodiaque. So, potentially everyone could develop Haki. However, the question of "Is this or that kind of Haki any use to the character?" remains. And right now, I don't see why any of the other Straw Hats should develop Haki.

Luffy, Zoro and Sanji have Haki simply because they are the Monster Trio. They fight all the strong guys. They need any batshit awesomesauce überbility they can get.

Usopp is a sniper. So being able to sense the aura of whatever he's trying to shoot is an invaluable ability to him.

But I don't see why anybody else in the crew would absolutely need Kenbunshoku. Nami already has her convenient weather senses, Chopper has a good nose and his Scope ability, Franky could have all the sensors he'll ever need or not need and Brook is able to slip through walls unnoticed already. I guess Robin could use it to extend her Hana-Hana ability. But it's not important for anybody else.

As for Busoshoku, that's an even more generic ability. Uh, you hit harder and have more defense. I guess that falls under Lame But Practical. Obligatory When-in-doubt ability. Hope you catch the gist. Again, Franky is basically a Pacifista and Chopper has his Plush and Monster abilities. Usopp is a sniper and Nami's fighting style is also long to middle-range and weather-based. Again, Robin and Brook are the only people for whom it makes any sense.