@Yvain: Mathematical proof is a valid argument, even if it doesn't contain any information, what was true remains so.

Fiction isn't supposed to act as evidence, it's supposed to place you in a specific focus of attention, where you resolve your own questions for yourself, from evidence you already hold. It doesn't explicitly state abstractions which you are supposed to learn in order to master new thoughts, reinterpret old data, or bind existing morals. It invites you to invent abstractions on a given topic for yourself.

Of course, all the usual biases will haunt you no less than in real life, plus the bias to interpret fiction as literal evidence, and they can be exploited to derail you just as it happens in the real life, only with more control. Although, control in fiction is still like programming: omnipotence without omniscience, where ability to manipulate the story doesn't always come with a way to efficaciously bias the reader.