I suppose I can understand the argument. I know if/when we find life on another planet or moon (Europa, Titan, etc) that no living matter from Earth should touch that world, for fear of mutating / polluting it. But when it comes to others we either know have no life (Mercury, Jupiter, etc) or most likely don't (Venus, Mars, etc), what is the harm in human exploration or even terraforming attempts?

Even if humans "infected" the whole solar system, how can we spread from there? The Centauri system is the next closest star system [1], most likely without planets there, and is over 4ly away. That's roughly 25 trillion miles away. If we could travel at the same speed as New Horizons (36,373mph) [2], it would still take 77.5 thousand YEARS to get there. [3]

We are not gonna pollute the universe and its a silly to think ourselves capable of such a feat.

[1] http://upload.wikimedia.org/wikipedia/commons/e/e9/Near-star...

[2] http://en.wikipedia.org/wiki/New_Horizons

[3] http://www.wolframalpha.com/input/?i=time+to+travel+24.7+tri...