Philosophy seeks to create systems of thought, to neatly explain reality; and usually operating from an armchair, with little knowledge of the real world, philosophers will try to fit a square mankind into a round system. So a few cuts on the edges will have to be made on Mankind in order to fit, but it'll always be a crude piece of work.
Fiction, on the other hand, shows that the world can't fit into any system. Fiction is chaos, discord, doubt, uncertainty, mockery. Characters do not exist to mouthpiece their creators' ideals. If that were so every novel by Dostoyevsky would be filled with orthodox Catholics who rspect the Czar's power. But instead they're filled with anarchists, nihilists and hedonists too.
And even if Dostoyevsky were using some characters as mouthpieces, the fact remains that a nihilist like Stavrogin, who embodied everything the author despised, is treated with much autonomy and actually ends up being more seductive and fascinating than the mouthpieces.
And what to say of the religious John Milton, who wrote Paradise Lost to justify the ways of God to men but instead created an enduring figure of rebellion - Satan.
Only mediocre writers like Ayn Rand use fiction for strawmen arguments. Real writers know that the world is above all a huge mass of uncertainty and doubt, and use dozens of fictional voices, contrary and contradictory, to reflect what a complicated, fragmented world we live in.
what about you?