Personally, my personal guess that some of us think of light, maybe its from the fact that the teaching of christianity is implemented to us worldwide (being a dominant religion). So probably some of the fanbase want to add the Light versus Dark concept of Roman Catholic/ Cristianity religion.
Again, a probability. I mean, christianity/Roman catholic is weak in Japan. And Gamefreak's religion is Zen buddhism. So I may say that they find Yang/Ying concept as equal rather than opposing concepts to the dominant Heavenly Light versus Evil Dark from most of us. Maybe America Change the Japanese Evil to dark in case of Overprotective Soccer moms selling back the pokemon game they just bought for their kid (I mean they are everywhere in both east and western culture if I recall).
I understand it since my country is very heavy on roman Catholicism and Christianity.
Personally, Shin Megami Tensei handles our beloved Light against dark concept better.
Then again back to the discussion, Now I have a good reason to hate All of the Rotom formes even more thanks to Parabola charge if they got it.