I'm not denying the existence of the dozens of round rocks out there, but I don't think they should carry the same name as the 8 large primary planets.
There are certainly a lot of round objects in the Kuiper Belt, and possibly beyond. Truthfully though, there is little support to consider them all planets. They exist in a field of a large number of icy bodies, and have unusual orbits. They were not formed from the same disc of dust that created the 8 large planets on the ecliptic plane.
Pluto was only a planet because it was the first body to be discovered beyond Neptune. If it were discovered today, there would be no debate about it, we know it's hardly alone out there.
Ceres, the largest asteroid, is round. It was discovered in 1801 and was considered a planet for a time. Science realized that it's in a field of asteroids, few of which are round, and it's rather small, and it was decided that it's not a planet. That's the same thing that's happening with Pluto today.
My point of view is that people still want to consider Pluto a planet, but don't want to include the rest of the round bodies out there. The number of those will inevitably increase continuously, as we get better at finding them.
You can't be selective in the application of the term planet, and that's what I have a problem with. If Pluto is a planet, then Ceres and all the other round bodies in the Kuiper Belt have to be too. If they aren't, Pluto isn't either.
So, to sum up:
1. Pluto is tiny, smaller than most of the moons in the solar system.
2. It's in a field of a huge number of icy rocks.
3. It was not formed from the same disc of dust that spawned the 8 planets.
4. It has an elliptical orbit not on the ecliptic plane.
5. It's the only one of its kind considered as a planet.