What's with adding curses anyway? I don't understand… It seems like they feel obligated to either add, or take away. Can't they just leave it the was it was originally? Is that too much to ask?
Well, sometimes dubbing companies and their scriptwriters want to have a little more fun with the series or make things seem a little bit more dramatic without losing the feel. (Uh…sometimes this doesn't work out, like in the Battle Royale manga by Tokyopop.)
I believe it was during the Black Chapters arc in Yu Yu that Yusuke fired out this absolutely hilarious line: "Son of a bitch, bitch, bitch [thinking pause] BITCH!!" Fullmetal Alchemist was also another series that had some extra swearing, and that turned out great also, (dramatic and comedic purposes included.) Honestly, though, it's not like the swearing goes beyond the PG-13 boundaries, so it's nothing too harsh.
I mean, sometimes swearing makes things a bit more gritty and real, and that's probably the effect most scriptwriters use when they see these translated scripts. Shrugs Who knows.
Hopefully, One Piece's dub will do the series justice, which I'm sure it will.