I personally see it as a cop out when an author leaves an essential element of a story up to the reader in a way that makes it obvious there is no purpose to the mystery other than as a cheap device to grab an audience's attention (someone earlier mentioned the ending of Castaway). But other authors intentionally leave elements open to interpretation in order to create a reality different from our day to day sensory experiences, and the intrigue builds from the new perspective and its implications (House of Leaves comes to mind). McCloud describes this effect on pg. 133, writing, "...a lack of clarity can also foster greater participation by the reader and a sense of involvement which many writers and artists prefer." The sense of living, rather than just observing, seems to make open-ended stories far more compelling.