User blog comment:Monkey.D.Me/Fan-Service OVER 9000/@comment-3125914-20121023113730

There are different types of manga, each one being adressed to a certain category of people based on age and gender. In order to make it attractive for the target audience, manga artists have to use certain elements, besides a good plot and good art.

One Piece is a shounen manga and it's also published in Shounen Jump, which means the target audience is represented by young boys. So in order to make it more attractive for them, manga artists add a bit on fanservice as well.

However, I have to say that it became exaggerated and less of that won't hurt. After all, the plot is what makes it fun to read.