This is an interesting point. However I would argue that you cannot have 'good' scenes without bad or less interesting scenes, or at least that the best scenes of a film cannot be viewed without reference to the piece as a whole. Being part of a whole, all the scenes in a film necessarily have a relationship with each other. These relationships cannot help but affect how we the audience interpret, receive and react to both individual scenes and the film as a whole. To rephrase your idea I would perhaps argue that a film is as good as the interplay between the most engaging and least engaging scenes. The Normandy Scene in SPR is incredible and undoubtedly steals the show, however viewed on its own it isn't the same experience. The scene invests you in the struggle of the protagonists and gives the rest of film more emotional context through its powerful and innovative depiction of the landings. Only when viewed all together does this scene give the film a warrant to be considered a great movie; without it the movie would fall flat, but on its own the scene would loose significance.

Now I take your point that the rest of the film is perhaps tonally different to the Normandy scene. However I would argue that if the rest of the film was consistent enough in quality that people still view it as a great film, even if that view is largely based on the strength of that one scene, it would be somewhat reductive to put it down to just that scene.