Propositions are moves in a language game that intends to describe things. They have truth value, in the sense that what they propose is meant to be considered as an assertion describing reality. But there are other language-games, which are not primarily concerned with this task.
Statements of possibility are generally not informative. They are moves in the game of thinking itself, or in some sub-game about thinking, like auditing understanding. They inject a parameter over the range of possible worlds, and request that one's listener consider whether the parameter with possibility has been considered adequately. In doing so, they generally put you farther from certainty (correct or otherwise) rather than closer to it.
They are not even informative statements about alternative worlds. When would one say "It might be dark."? Not when one thinks this is information the listener does not have. If he did not know of this possibility, he would need a lot more information about the nature of light, in order to care. And any of that information would be more useful to contribute. Instead one says this when one thinks the plans so far do not adequately consider that dimension.
Since they do not cover descriptive ground, but instead open new ground, they do not contribute directly to understanding in the same way. So such statements do not play the role propositions are meant to fulfill. It is possible to interpret them as propositions, but doing so does not allow them to perform their intended function. If someone contributes "It might be dark." and your only response is to affirm the possibility, he has failed to communicate.
(Answering Pacerier's silly question, if Mary asks whether it is dark outside, and John replies that it might be, he is agreeing that he had not to that point considered whether or not it was, and whether it should have any effects. He might only be so agreeing in excessive politeness, if he is fairly sure it does not matter.)