You forget that, in Japan, the word for the actual Dark
type (in the games) is the word for Bad/Evil as well, and yet the TCG still felt the need to make a distinction between Dark Type and Dark Pokemon, in the same way they could/would have if there was a Light
type to go with the Light Pokemon.
More importantly, so many people against the idea of a "Light" type oppose it strictly because we "already have a counter to Dark" and that's simply not the case.
First is the obvious fact that Psychic is
strong against Dark and thus, not its equal. A theoretical "Light" type would have to perfectly balance the Dark type, and I don't mean "be strong against it". I mean actually balance it - like Light and Dark, Good and Bad, Yin and Yang are perfectly balanced in real life. To have one side stronger against the other would be giving bias to either Light/Goodness or Dark/Badness, and doing that would be based purely on religious biases and Pokemon doesn't do that. Light and Dark would have to either
both be super-effective, ineffective, or neutral against each other - any other way wouldn't make sense. That way the delicate balance they've created with Dark and the other types wouldn't be harmed. If they really wanted to "balance" things out though, they could make Light weak to Poison and Psychic and strong against Fighting. For one, Poison needs a new strength desperately and by making Light weak against Psychic but strong against Fighting a perfect inverse of the Dark-type could be created - an idea which I think would be very fitting.
The second point is the fact that Psychic is simply
not a substitute for "Light" type. If we're going with the assumption that "Light" means goodness and
not luminosity or the sun, then the
only Pokemon that "should" have been Light type (as of generation 2) are Togepi, Togetic, Chancey and Blissey due to their clearly goodness/kindness-based themes - not Psychic types. So with only four Pokemon holding the idea of a Light-type back, this is hardly any different then the Steel type for Magnemite and Magneton. You might argue that all the references to the Sun should have been Light-type too, but have all the references to the Moon been Dark type? (Answer: No - Cleffa/Clefairy/Clefable, Lunatone, Cresselia).
If they wanted to introduce a Light-type, it wouldn't be that hard. Leave all the existing "good" Pokemon alone and just change the typing of a few older attacks - preferably non-damaging ones. I think a little bit of change would be good for the series, and could lead to a lot of really neat evolution lines. I mean, look how many Pokemon evolve based on kindness (but oddly not from frustration) - I think by diving deeper into this whole good/bad, light/dark thing they could really create some interesting and dynamic gameplay mechanics and new Pokemon.
I just don't understand the hate!
-Age