First of all, I feel obliged to say that I am practitioner of traditional approach and I am not here to defend modern astrology. I don't think that "outer" planets co-rule the signs. But also, I don't think that they are completely meaningless in astrological sense.
Some things said here are simply not true.
-Pluto doesn't reflect light:
Uranus and and Neptune don't reflect light too- they are too far from any source of light, e.g. our star Sun.
Every object in the Universe reflects the light (except black hole maybe) and whether something has the property of light reflection or not, has nothing to do with the distance of the light source. Otherwise we wouldn't be able to have photos of those planets. That is what camera does: captures the REFLECTED LIGHT.
In this photo you can see light reflected from the planet Uranus.
-You can't see Pluto with naked eye:
Uranus and Neptune are also visible only through telescopes.
Uranus can be seen with naked eye. Here is an instruction how to see it yourself:
http://www.space.com/22983-see-planet-uranus-night-sky.html
Of several schools of traditional astrological thought closest to my personal understanding is Stoic view: Planets do not cause the events, planets are objects (or even beings) which
SIGNIFY things. According to this view astrology is a
science of interpretation and is divinational, not part of the physical sciences.
This is in total contrast with Aristotelian-Ptolemaic view in which planets
CAUSE things to happen, they are causal agents, therefore astrology is virtually branch of physics, and in some sense part of natural sciences.
Ironically enough, latest scientific developments continuously confirm holographic nature of our Universe in which every part contains the same pattern and reflects all other parts in itself. This is perfectly aligned with Stoic view of astrology. It is not the size of the planets, nor their gravitational field, nor property of light reflection, nor our ability or disability to see them with naked eye what makes astrology to "work" but their configuration in the skies which is mirror-like reflection of people, things and events (under assumption that the interpretation is correct). As above - so bellow.
I completely agree with KnS.
Another issue with the outer planets in general is that they lack much of the tools that the classical planets have. This isn't just referring to dignities (though that is a large part of it), but they also lack nature, sect, gender, years, winds, orbs, signatures, etc. This may all seem superfluous or unnecessary, but its significance really cannot be overstated. Without these associations, the outer planets are essentially blank orbs without instruction or meaning.
But it is important to remember that those planets still SIGNIFY things (and believe me, my most accurate predictions in sport astrology are based on them) - because
everything is the reflection of everything. What exactly they signify we cannot be certain at this moment. Astrologers had observed traditional planets for thousands of years before they managed to establish their significations. So let us, in the same manner, observe "outer" planets at least for some time (I am talking hundreds of years) before we dismiss them or accept them.