That's a good point. However, I think it's possible that the film is playing with the idea that the human condition is best defined by the infinite need for possibility; Bruner's closing statement is about the importance of possibility, and yet I actually think that's the central theme to the movie.
Though evidence in favour of the supernatural is almost overwhelming, technically the movie doesn't do anything that can't be rationally explained; examples include that the 1 to 6 count Bruner hears on her way to Moore's cell could've been Moore himself whispering the numbers on purpose, or that Cartwright seeing the demonic can be attributed to him earlier telling Bruner that he's begun praying again - this implies that Cartwright is inherently religious, therefore susceptible to irrational thinking.
And that's where it gets really interesting, because one can take the view that if the movie's about the morality of belief, it's doing things that are unnecessary! It doesn't need to play internal games with its audience, just in order to promote the idea that faith in the supernatural is righteous, and so perhaps the ultimate question is whether all of what Exorcism of Emily Rose does is helpful to the agenda of promoting spirituality.