The sloppy parts of the definition ("sloppy" is the word used by Alan Stern) are: a) the vague concept of "clearing its orbit," which if applied literally, could exclude all the planets in our solar system because none fully clears its orbital fields of nearby asteroids, and Neptune does not clear its orbit of Pluto; b) the definition of a "dwarf planet" as not being a planet at all.

The further an object is from its parent star, the less likely it is to clear its orbit. This means if we find a Mars-sized object in the Kuiper Belt, according to the IAU definition, that object is not a planet while the real Mars is. There is an obvious problem here in that the definition looks solely at where the object is rather than what it is.

Geophysically, Pluto and Eris are much more like planets than like asteroids. Yes, they are different from the other eight, but that simply means they fall under a different type or subcategory of planet. There is no ambiguity that these objects and Ceres have in fact achieved hydrostatic equilibrium.

Astronomical definitions of other objects, such as stars and galaxies, cover a very broad range including many subcategories. Stars are not demoted because they are part of binary systems and therefore don't "clear their orbits." Tiny dwarf galaxies orbiting with the Milky Way are still considered galaxies even though the Milky Way could be said to control their orbits. Why should the definition of planet be so narrowly limited in a way that other astronomical definitions are not?