I'm with the other guys. Some of the types we have now covers light, and they create light. Flash is one example. If it was there, then there wouldn't be much Pokemon with the dual types anymore. Celebi would be Grass/Light, and as you stated, Reshiram would have been Dragon/Light based on the origin.
And I don't think that Light and Dark are 2 representations of evil for Pokemon. Pretty much, they're just elements that cover the Pokemon's category. Psychic is like light in a way, since it has to do with warping time or using extrasensory powers. It's like light, but in the opposite way of Dark.
In other ways, we have types that cover light and it's unnecessary for that. It would change a lot of Pokemon's types, and I don't think that it's good for normal Pokemon to have these.
Personally, it'd be kind of confusing; a number of older Pokemon would change types too, therefore you'd need to get used to type matchups and stuff again.
So, I think it's for the best, although a new type would be cool, to leave things as they are.
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I never for even a second thought that Reshiram might be an introduction of a Light type, since I never for even a second thought, or think, that a Light type would be, or will be, introduced. I think GF didn't do it because they have no intention of doing it.