Funny, but I think the actual concerns are more along the lines of "What happens if it works?"
If they accidentally unleash an Earth origin extremophile, it would have the potential to taint any any life that may already exist on Mars. This hypothetical life on Mars probably isn't of any real use to us, obviously it isn't megafauna or something, but it would be of unspeakable scientific importance. The chance to study live from an independent instance of abiogenesis.. I just can't find the words to describe how important that would be.
We know all that. That's why we're being cautious about exploring it now; but we should be able to get a definitive answer on the presence or absence of Martian life within a decade or two. If there is life there, we'd be better off studying it iin situ than trying to transport it back and forth.
Present or not, we need to have some kind of strategy for how to survive there on a long term basic, either for a small-scale scientific mission or on a larger scale to terraform.
Besides, there are several other places in the solar system that have the potential to host life, such as Europa. I don't want we should spend so long sitting on our hands that another generation or more has to die without seeing the human race explore further. It irritates me that it's already 40 years since the last moon landing: http://xkcd.com/893/
Oh certainly, I feel the same way. We are definetly going to put humans on Mars eventually, and better sooner rather than later, but I think that it is something that needs to be done with an incredible amount of caution. There are definitely concerns about contamination that need to be considered carefully when planning such missions.
That is certainly a possibility. However contamination of Mars will make coming to that conclusion, if the evidence points to it, a great deal harder.
(Well, obviously contaminating Mars is itself panspermia, but not really of the interesting sort. A confirmed instance of "natural" panspermia would tell us a lot about what we can expect about the distribution of life throughout the universe.)
If they accidentally unleash an Earth origin extremophile, it would have the potential to taint any any life that may already exist on Mars. This hypothetical life on Mars probably isn't of any real use to us, obviously it isn't megafauna or something, but it would be of unspeakable scientific importance. The chance to study live from an independent instance of abiogenesis.. I just can't find the words to describe how important that would be.