"f you define "planet" (the non-dwarf kind) to include Pluto, then you need to also call Vesta and a bunch of other objects "planets", so instead of 9 planets, it'll be somewhere in the teens, and growing as we discover more Kuiper Belt objects."
and that's bad because.....
In the end a rose by any other name is still a rose, but the idea we knew the mass and approximate size of this body for so long and called it a planet and then decided it had to have a modifier because we found more is sort of dull.
The whole concept of a planet having enough mass to pull itself into a spherical shape seemed to be a reasonable dividing line because it seems silly to call every chunk of matter in orbit around the sun a planet. Having to have enough mass to toss all other bodes out of it's path just seems arbitrary.