There are two types of people in the world, artists and scientists. Both are fascinated by the mysteries of the world; things that seem inexplicable. Scientists will seek to find an answer, discover why things are the way they are. Artists revel in the mystery.
In the fifth grade I had a teacher who was more than a bit of a science fiction fan, and spent an inordinate amount of time on stuff like the Bermuda Triangle (why would any fifth-grader need to know about it at all?) and the theory that aliens built the pyramids (we actually spent class time watching a video on this). What I came away learning from this is as much as we understand some parts of the way things are, we really don't have a God damn clue.
Call me lazy if you like, but I'll gladly revel in the mystery.
The thing is that there's very little that's "ambiguous" about these things. There's a lot that isn't explicated, but you always end up knowing everything you need to know. Anything left out is not only unessential, but made more tantalizing by its omission.
I once got into a heated debate about these sorts of endings, specifically as relates to Martin Scorsese. A friend claimed his movies didn't have endings, that "they just stop." I always wonder what one would have to gain by finding out whether or not Mark Wahlberg pays for what he did at the end of The Departed, or what happens after the screen goes to black at the end of Raging Bull and The Aviator (did we need to follow these people until they died?). And what's up with the ending of Taxi Driver?
In his commentary track for L'Avventura, Gene Youngblood mentions that he's amazed at the number of films that tout themselves as mysteries when they end up simply revealing everything, and he's right - I'm far more thrilled by the questions swirling around in my head as the final shot of L'Avventura plays out than I am by hearing the psychiatrist's explanation of Norman Bates' condition at the end of Psycho. As thrilling as that film otherwise is, that brings all momentum to a complete hault - how much more satisfying would it have been to simply cut to the final sequence of Norman sitting in the chair?
Art leaves us asking questions, or at least it should. Leave the answers to the sciences, but only when we absolutely must have them.