I'm sure that people stopped listing Ceres, Juno, Pallas, and Vesta among the planets of the solar system long before 1957. All asteroids are still also called by the name "minor planets", thus their orbits are listed in documents called the "minor planet circulars", but the Solar System had nine planets, not thirteen, after Pluto was discovered.
Pluto got demoted for the same basic reason as Ceres and company - there were too many others just about like it, starting with Eris. I'd prefer Pluto to stay a planet, but I really can't come up with a convincing argument against this brute fact. If planets aren't clearly bigger and more prominent than non-planets, the term loses its meaning, and if there are hundreds or thousands of planets, that's a useless situation too.