There should be a better reason then it's the way it's always been, it's always worked so far and therefore is part of our culture. Can you imagine if our ancestors said that about switching to iron working, or agriculture. That said I'm not trying to be antagonistic and acknowledge this is a matter of personal preference. I understand that many people do prefer open-endings as they would rather end the story in their heads and that is a fine viewpoint. I personally view it as lazy though as if I wanted to make my own ending why would I read a story in the first case? I could just imagine my whole own story and cut out the middleman. When I read something I want to read the authors story and if they haven't finished the story then in my mind it is incomplete.
That is just my viewpoint though, this is probably a point that could be argued for a while with no clear end in sight. It isn't terribly important regarding dragon commander anyway. I felt they had a perfectly resolved ending. Closure was given and all loose threads were in fact tied up. What I wanted was an epilogue, not a new ending. I usually want this as when writing is as good as DCs I want to hear more about how the characters I know and love ended up and an epilogue gives that.