Since dark type represents "evil" more than actual darkness per se, it actually makes sense that the legends aren't light and dark. To be a true counterpart of Dark type, Light type would have to represent goodness and purity. However, in a Yin-Yang, neither side is better than the other and both are needed for balance. It is a far cry from the concept of good vs. evil. The naturalism vs. industrialism that fire vs. electric represents fits the symbolism of the Yin-Yang much better. Thus, although a light type is unlikely, the fact that Reshiram is not Light type has no bearing whatsoever on its inclusion.
I actually wouldn't mind a light type, but I won't be too disappointed if it never showed up. I do think it would be relatively easy to add without unbalancing the game or making it any more complicated than it already is, though.