As with many anime, one of the main problems present with One Piece is the overwhelming amount of filler it has. Of course its wonderful to get an episode every single week, but those episodes aren't always relevant to the story line, or fulfilling viewing experiences.
Having more chapters of the manga available for animators means they get to pick and chose which parts of the story are most important to tell. Oda's world is expansive and fleshed out well enough that episodes that don't further the plot are detractors, not additions to the story. Having the source material available well in advance of animation will make for a more concise and action packed viewing experience for fans.