Pluto hasn't been "demoted" exactly; it is still technically a planet. What has happened is that the IAU created a medial category, "dwarf planet", to comprise all those objects that were too small to be proper planets yet too big (read: round) to be mere rocks. |
The decission restricts official use of "planet" to the Mercury and the ones that are bigger. Pluto-Charon is joined (quite logically) with Ceres, "Xena" and the other objects that are round and orbit the Sun but have other objects in their orbit. They call them "dwarf planets", fine to me, but it's still a different cathegory than that of planets.
Regarding Astrology, I agree that Sun and Moon are not planets but they have an even greater influence, at least gravitationally, on Earth. And standard Astrology is geocentric. You can't compare the Sun with Pluto, even if Pluto is still meaningful to Astrology. You also ignore the Moon in Heliocentrc Astrology.
For me it also raises the question of Scorpian rulership. The Pluto-Scorpio connection is quite clear yet, must we consider it the ruler? Or should we consider Mars again as that? Or both? I'm amiss.