Urhh... in "One Piece world Z."
Luffy, Sanji and Usopp wore coats in the costume that was clearly designed just to sell action figures laters.. and they only wore them for a few minutes before shedding the coats..
in Punk Hazard they all wore coats... except Smoker because he's a vice-admiral now, they only do cape coats... Law! Law wears his coat post-timeskip.
It is a hillarious notion tough, you are right! By far most of the characters in one Piece like to hang their coats on their shoulders like a cape, and then that's just it.
My big question though is... how come this trend hasn't caught on in the real world yet? Where's my magical coat that will just hang unto my shoulders like that?! so I can look badass and pretend to have a cape without being to dorky about it.