It really isn't a planet. It's a comet, and more specifically a Kuiper Belt Object. Compositionally, dynamically and formationally, it is vastly different from both the terrestrial and the gassy planets. It instead fits in with comets and its fellow KBOs, just like how asteroids fit in better with fellow asteroids than with the terrestrial planets. In my opinion, Mercury should also be demoted. It is much more like an asteroid than the other terrestrial planets and should be classified as such. That's not to say that there isn't much to be learned from these bodies. As this article describes, there is a lot of really interesting science that they can reveal. They are the largest objects in their different non-planet groups, but that's what makes them so interesting. They aren't planets. They're something else and that means that they can tell us things that planets cannot. But they shouldn't be considered planets because they aren't.