Well, regardless of their political views, their reasoning is very much grounded in actual astrophysics. Planets form by sweeping up all the material in their respective orbits, more or less dominating them. Pluto formed in a manner more similar to asteroids in the asteroid belt. It has far more similarities to them and other KBO’s (Kuiper belt objects), so it’s not a far stretch that it deserves to be classified among them. But we’re still discovering things about it, so who knows...
The four rocky planets -- Mercury, Venus, Earth, and Mars -- obviously formed differently from the gas giants -- Jupiter, Saturn, Neptune, and Uranus -- hence, there is no astrophysical reasoning behind it, and as you said, more will be discovered, so why the rush? The ONLY reason for the reclassification was the blind panic over US planetary discoveries. Mike Brown, who agrees that Pluto "had to die", but who was thus denied discovery of a tenth planet, also saw the creep from Spain get off without censure or any other symbolic punishment for hacking and stealing Brown's research. Obviously the hacking and stealing was a real issue that should have been dealt with, and was ignored instead.