Certainly I agree with you on The Pianist and I also agree with you that a voiceover is not necessary in general and in some cases is downright redundant and irritating. However there are cases in movies when a voiceover adds to the style which I have mentioned before, particularly in an adaptation of a novel which keeps the literary style of the movie, as in Tony Richardson's classic adaptation of Fielding's Tom Jones. To say that a voiceover is not called for in The Pianist you don't have to go overboard and insist that voiceovers are junk. Sometimes they are an essential part of a certain movie's style.