If there's anything that will put me off a novel or book series, it's loose ends. I want every question to be answered, and where appropriate I'd really like to learn the ultimate fate of every single character, ideally. And that's why I don't truck with stories that contain unresolved subplots. Here's why it ought to matter to you: I'm not the only reader who feels this way.
But What About the Dog?
Subplots are used to add meat to the main story. They're around to provide excitement, or perhaps mystery, maybe even romance. In the best stories, I find, the subplots are intricately woven around the main plot and all the threads interconnect somehow. When subplots are just hanging out there for no reason, I always feel a bit like my time is being wasted so I appreciate it when everything ties together.