, which is possible
, would be a good thing because it would be a good sign of a high probability of finding life
elsewhere in the Universe
Human exploration of Mars followed eventually by colonization and perhaps terraforming, would be a good thing. Having all humans living and being dependant on Terra is very risky - one well placed catastrophe could easily eradicate all known life in the universe. Having independent extraterrestrial colonies reduces this risk.
The problem is that if we find life on Mars (a good thing), we would not be able to send humans to explore, colonize or terraform (good things), because that would destroy indigenous life. Even if the life was only at the bacterial level, we would still morally not be able to use Mars for our own purposes. Thus, two good things (possible life on Mars, possibility for human inhabitation) conflict to form a bad thing.
Which do you want more - life in the universe, or the survival of the human race?
I first came across this idea in the short story Discovering Life in the Vinland the Dream short story collection by Kim Stanley Robinson.
A SpaceDaily.com article from the 1st of August 2003 discusses this issue. It may be found at http://www.spacedaily.com/news/mars-life-03e.html