Since so many people think it didn't add anything to the movie, I think it is a failure on the part of Coppola to communicate. Especially because there is a big significance for the scene in his mind, as he explains it in the commentary. The whole movie has "going back in time" as one of its many themes. If you look at the beginning, you see choppers and gun fires etc. then we see bows and arrows and palm leaves for the boat etc. Then in Kurtz compund, we see even more primitive world. In essence, he wanted to show a gradual shift in time backwards as willard and his team slowly drift towards Kurtzian morality themselves. That is where the French Plantation scene is important. First of all the french are living in the past in vietnam. Some kind of time bubble that is not real. it is interesting to visit this time bubble in the backward journey through time. Secondly, we see the burial of "Mr Clean" with all state honors. This serves as a contrast for the way "chief" was treated upon his death later. Thirdly, the conversation between Roxanne and Willard is very profound. She reminds him that all that matters is that he is alive (her husband isn't, because of the war). At this point Willard has lost all purpose behind his mission. He is no different from Kurtz or Killgore. His journey has taken on its own course by then.
Knowing these facets of the scene makes it very enjoyable for me each time I watch this scene. When I watch Redux, I really don't care that it is long and meandering. It is an experience and not a movie at that point. I compare it to the movie "Dead man" which is again about a man's jounery into himself. I want it to go on and on forever.