Its a good point. Given this is targeted at the naked eye discussion, I will keep my response towards that first, and then I'll move to the outers. I apologise because it may be rather long.
The light issue is part of the symbolism that is intrinsically bound to astrology. And the astrologer uses that symbolism as a way to make a proper interpretation of the message. Think of this:
Why is it the Ascendant the most important house in the chart? In an ephemeris the Ascendant is nothing more than another house in an equal zodiac circle. But by looking at the sky, it is the place where dawn occurs. The house marks the beginning of the daily life of the Sun, it brings forth daylight rescuing us from darkness. This expresses the attributes usually given to the 1st house, through the myth that occurs in the Ascendant. This symbolism is also seen in the 10th house, the second most important place in the chart, by symbolising the zenith of our life (our rank, what we achieve in life), which is represented by the Sun achieving its maximum point and rising the temperature to the daily max. This example is in my perspective one of the reasons as to why the argument for the ephemeris doesn't really work, because the ephemeris is a tracking system for the mathematical positions of the sky, not for the symbolical one.
Similar symbolisms are attached to every house, to every planet, to every sign. One of them is the heliacal cycle, and the movement of the planets in relation to the Sun. The concept of birth/rebirth of the planet is embedded in that symbolism. The best example is Mercury, the "hermaphrodite" planet. The reason Mercury is considered to have that duality is because it often swaps sect, by its short cycle of death and rebirth. However, if you remove the symbolism of transformation there, you remove the concept from Mercury, after all Mercury never "dies" and is never "reborn" into the other sect.
Other planets have similar sectarian benefits, such as Jupiter and Saturn, considered diurnal planets rejoice when oriental. Why is this? Well because by being oriental, they reinforce their masculinity, they announce the Sun's arrival and herald the day. In contrast feminine planets rejoice while occidental, when seen after the Sun has set, being bright in the coolnes of the night, their sect.
In the practical sense there are a number of techniques that relates to this concept of visibility. For example, the fixed stars we usually focus on are the brightest ones, such as Regulus, but we rarely care much for the dimmer ones. Yet, in an Ephemeris, Regulus doesn't really stand out from other stars.
It also seems that luminosoty might be related to the concept of benefic and malefic, although this is just a speculation in some traditional circles, and I do not recall any actual data from the ancients. But I'll add it: from the 5 errant stars, the 2 brightest ones are the benefics, and the 2 dimmer ones are malefics; the same way bright stars such as Regulus or Spica are considered to be beneficial stars, while most malefic stars have varying or dimmer magnitudes.
Ok, so then whats the deal with the outers? The outers because of their lack of visibility do not fit into many of these myths and symbols. They are rather incomplete in our view, in fact less complete than fixed stars. In many modern circles they are treated as "2nd class" planets on equal level with fixed stars. The only thing different is that we simply take a step foward and reject them all together, which isn't much different.
But why do we reject the outers being seen with a telescope? Well first, because it doesn't fix the symbolistic problem they have regarding many of the techniques, and second, at least for me the concept on involving technological development and scientific advancement into the scheme of astrology is just a matter of "how much" you wish to include, which seems rather picky: just enough to make a case for the outer planets, but not too much as to deny astrology all together. Earlier we briefly discussed the causality of astrology and (in a way) the order of the cosmos. Science has a different view than astrology regarding the origin of the universe, free will, etc.... yet we do not include those discoveries into our "theology". The reason is simple, it opposes it.
Sure you can use a telescope to see every actual celestial object in the solar system. But we don't include all, just some. Yet do we take pluto as a giant asteroid like science dictates? We do it for Ceres who is similar in size, but we give pluto the treatment of a planet. So we only use the scientifical concepts we wish to use, but we deny the rest. And yet, the telescope can't add this symbolism into the planet. Pluto can't be seen as such.
That is at least for me, the reasons why I don't see a case for the outers through telescopic use. Maybe I am wrong, who knows?
Regarding the future, I have never denied the possibility of them becoming important if, for example their orbit changed and suddenly they were able to be seen. Maybe that will happen. But for now it doesn't, so to me, they are non-existant within my astrological scheme.
Sorry for making it long.