I think this has become a modern trend especially in long-form art (television, book series), where it's been noticed that mystery plots are a good way of attracting and maintaining an audience with much less effort than crafting a satisfying narrative takes. That's how we get things like Lost, Battlestar Galactica, or the Game of Thrones TV show (perhaps the ASOIAF books as well, but time will tell), as two prime examples of captivating the audience without a clear plan.