Certainly, that is true, but I think there is a qualitative difference to the sorts of questions asked and why. For example, in Inception, no one in the movie asks if they are dreaming when they think they are awake. It is a suggestion in the movie by Mal, but no one really thinks that. Yet, the question does plant a seed (incept an idea) in the audience that resonates to the end, and that question then spills over into your life in an unexpected way. Not - am I dreaming when I think I'm awake, but more, are the choices made and actions taken in dreaming any less important than when we are waking? Do the internal consequences of conception - our thoughts, dreams and fantasies - matter less because they have no direct real world consequences or does the existence of imagination guide the effect we all have on our world?
A different type of discussion. The Avengers has people saying, "Did you laugh at the Hulk? Wasn't Hawkeye badass?" Promethus has us (some of us) saying, "Do you think...?"
Even you, in your it-was-terrible posts are still moved to discuss the points you claim the film itself doesn't address. If it can get you talking about them, it doesn't actually need to answer them itself.
So, the question "is Cobb dreaming through the film or at the end?" which is never actually asked by anyone in the movie finds attachment to a lot that goes on outside the movie.
The same for the question whether or not Deckard is a replicant and does not know it in the original theatrical cut of Blade Runner. The story, by not directly asking or answering the question, leaves more of an impression that makes it more significant to the viewers. A question, ironically, Ridley Scott answers in later and in my opinion lesser cuts of the movie.
In Prometheus, the reasons why the Engineer made us, why they created the Xenomorph and why they wanted to destroy the human race on Earth are interesting to speculate about, but they don't escape the confines of the narrative. They have not become intrinsically significant, and I think that is due to the carelessness of the storytelling and insignificance of the characters' actions.