The problem with Pluto being a planet is that, if Pluto is a planet, then we likely have 100's, possibly 1000's of planets orbiting the sun. Pluto is considered to be of the same materials of the other objects in the Kuiper Belt; therefore, it is classified as such.
Not only that, but if Pluto is to be classified as a planet, I could argue that it shouldn't be the 9th planet but the 10th planet. Ceres is a round dwarf planet in the asteroid belt not that much smaller than Pluto.