I think spreading life to a dead world is a great thing. If it turns out Mars is lifeless, lets spread some life on it.

If Mars does have microbes, should humans still pursue terraforming the planet? After complete study, and maybe protecting some of the microbes (a microbe zoo) I think yes, because if there is life only microbe life on Mars, it is not evolving. If they have not evolved by now, with the Sun at half-life, then I doubt they will evolve before the Sun dies. Why let a good world go to waste?

Here is a thought; terraform Mars for the Martian microbes, if they exist, so they will start evolving.

As for the landers, one day they will be historical sites. Some will probably end up in Mars or Earth museums, especially if Mars is ever terraformed. I believe most of the landing sites will be under water if Mars is ever terraformed. The Landers will be valuable historical artifacts. Maybe they can pluck some of the orbiters out of orbit before their orbit decays.

As for polluting Mars. We are talking a few probes on an area of land equal to all of Earth’s continents.