I'm not sure that the distinction you make between scientists and other authors is as binomial as you suggest. In each case we are referring to actual people who have biases and intentions with which they will imbue their work. You have no more reason to trust a scientist simply because they say they have discovered X than you have to trust Marx.
What you trust (or don't) is rather the accumulated mass of people who also trust. We trust that scientists have indeed discovered something because so many of them agree, and so many non-scientists also trust them.
History is littered with the case of the single scientist having apparently discovered something new (using perfectly scientific methods), but whom the world only reluctantly believes until a critical mass of followers is gained. It is no different with Marx, or Smith. Had you never heard of them, but simply picked up one of their pamphlets on the street you would be no more compelled to take their arguments seriously than you would anyone else's. It is the critical mass of people who support them that give them whatever "power" they have.
Since, as was pointed out in comments, their arguments act, not so much as persuasive tools, but as justifications for already held beliefs, they act more as cameras, taking snapshots of society's beliefs at the time. Sartre didn't create the cafe culture. He was born from it. Neither did Marx create the working class struggle, nor Smith the Scottish enlightenment.
By and large the "tremendous consequences" you are concerned about have already happened, or at least been set in motion, by the time these authors wrote their pieces. Had they not captured the Zeitgeist, someone else would have.