As far as I can see, Pluto is an astrological planet because modern astrologers use it.
PTV says that the ancients used the outer planets, including Pluto, up until 12,500 BCE, and cites Edgar Cayce's channelled material, the channelled material of a woman psychic PTV knows personally, and the Vedas (but no specific passage) as evidence. At least there's somewhere to look in the public record even if you disagree with his theory, so thank you for giving some pointers in the direction, PTV.
Muchacho says that history is suspect based on his reading of chronological theory by a Russian mathematician, Anatole Fomenko, and that in reality, traditional astrology is a new-ish invention, and the meanings for the traditional planets were cribbed from modern astrology and the outer planets. Muchacho cites intuition as his source. That really can't be referenced.
A number of people say that modern astrologers have extensively researched Pluto, and Caprising says that he (or she) has researched it in his own work. Waybread cites Rob Hand's text, Planets in Transit. Rob himself has since recanted that text, as he discovered that in his own practice transits mostly didn't work, and that the language of modern astrology is so vague it's difficult to tell what's indicative astrologically when something is happening.
Waybread also says that Pluto 'works' as a house cusp ruler, but in horary astrology she uses Mars as the ruler of Scorpio.
Your reporter, Odd, has a thing for language, and would emphasise what Rob says about the imprecision of language in modern astrology because he believes that astrology is a shared human endeavour, and it's important to be able to use language to share it.
A good number of posters use Pluto because they say they feel it in their charts. Nobody has said what Pluto feels like, though, and several people have said that you must intuit that. This is problematic on a number of levels, to me at least.
Several people have noted that modern astrology is a psychic tool and not a science. If this is true, it certainly gives more latitude to do what you want. It also contributes to the problem (as I see it) of not being able to define astrological terms, like....planet.
One poster says Pluto is responsible for metaphorical death, but not actual death, since it rules transformation, another says it rules death proper, one says it rules notoriety, another says it rules wealth. Psychological suppression, child abuse and pornography, as well as eruptions, are given to Pluto by at least a couple of posters. The word 'depth' gets used a lot in connection with Pluto as most posters seem to feel there is a connection there, though it remains unexplained. All of these things can be accounted for using the traditional planets, though, so - why Pluto?
I really think it comes down to 'because I use it'. And you can if you want to. But you can certainly read a chart without it, too.
I know I'm biased, as is everybody, but I think this is pretty much what it comes down to, and I have tried to be fair. If I have grossly misrepresented you - please yell.
