my avatar: warcraft 3 phoenix summon... it is a phoenix, but not from AoM ;p
well let's start with the simpler arguments first...
regarding dark being strong against psychic... I guess that they must be thinking that their minds are so "dark" that it's "unclear" (dark isn't evil... it just means mysterious in this case) that even Pokemon with psychic powers can't detect their thoughts, thus resulting in the advantage... but then again, as if bug being strong against psychic made sense anyway... or psychic against poison...
Psychic is "originally" (and according to literla definition) power of the mind... but then the invention of cosmic power certainly linked Psychic types to be in control of any mysterious powers. And how does running faster (agility) part of the 'mind?' Agility sounds to be very physically related to me... yet it's psychic o.o;
Now the dark vs light thing...
First of all, holy IS are religious terms, as no one is ever flawless and perfect unless it's a higher deity of some sort. The term "holy" is sometimes used to describe a person only in the sense that "as if a higher deity is guarding this human being." Holy is quite strictly religion related only. Not that this relates to Pokemon or anything, but I just want to point that out.
Dark is not the opposite of light. It is a very confusing matter, and what you say does seem to be true, except one scenario is forgotton. We always say that darkness is the absense of light, where light is the absense of dark. It seems that they define one another, and therefore, must be opposite, like "good vs bad, righteousness vs evilness and etc." However, try lighting up a match in a dark room, then you see both light and dark... Light has a strange property that "darkness" do not have. Light can exist in a room of darkness. In a dark room, you will see light even if the light is dim. However, in a room of complete darkness, you cannot possibly have "darkness" without removing the source of light. Therefore, from this simple scenario, we solve the impression that light doesn't "equal" to darkness in every way, as light has different properties compare to darkness in some special cases. So, just because one exist, the other one doesn't necessary have to exist, because not all the time do they share the exact properties the other one processes.
Another example: your eyes. In a room of "darkness," sometimes you cannot see anything... however, after awhile your eyes will adjust, and will pick up the faint and dim amount of light in the room and you can still see something at the end. There are of course cases of truly complete darkness, but the point is: what appears to be complete darkness, still has pieces of light within. However, in a room filled with light, you cannot possibly find darkness except darkness is hiding... so another example where they do not share the same properties.
Light and Dark = "Good vs Evil?"
On top of what Pine said, it is correct... we do associate different things with different images due to the past. However, I wonder why the chances of meeting a theif in the nighttime in a dark alleyway is much greater than meeting a theif in broad daylight in the middle of Wall Street? There are times where those "stereotypes" are drawn quite correctly. Even though over-generalization is involved, I'll rather believe in those stereotypes and not walk around in dark alleyways at night in order to be safe you know? Plus, the fact that "white light" is the absense of all colors, while "black/darkness" is the mixture of all colors are symbolic to this belief too... white light are "pure" as it really reflects all colors, while black/darkness absorbs all which reflects it as a mixture of chaos... this symbolism also supports that by auto-default, dark = "more evil than light", and white/light = "better than dark."
If they don't share the exact properties, then do not try to draw a parallel between them... And if we've come to the agreement that "righteous/good vs evil" shouldn't be involved in Pokemon, then leave light types out of this... or at least, reword "light" type to something else...
*too tired at night... didn't proofread and not sure if this made sense... i'll check... tomorrow*
