"The one being the fact of an enormously successful bacterium that lives in earth's oceans.

The other being the supposition that rugged life from Mars might be hazardous to earth life.

Was there a step that I missed? "

The mars bug would be like terrorists- they are unable to win.

[note: I'm sort of kidding:]

[And Btw don't think there is a significant threat of mars bugs affecting Earth's eco system, but I would suggest that there a lot we don't know about microorganisms and the microorganisms "rule this world", and I would say the biggest danger [though chance of danger is very small] from a Mars bug, *might be* it brings new tricks which have a slight chance of being incorporated by earth bugs. And I realize this is fairly speculative, but the danger is in what we don't know, and for that reason, I suggest this possiblity. BUT there is the possiblity that Mars bugs could have arrived on Earth billions of years up to the present- in fact I think it would very likely [thereby nullifying the whole possiblity of there being a threat], ASSUMING that there is life on Mars in which we capable of running across {though not if life on Mars has been and is rare and in isolated areas {which I happen think is the mostly likely case}}. Finally, in practical terms, if we are spending billions of dollars to find life on Mars, for this reason alone, we don't want Mars samples to be comtaminated, so the precautions taken for the cancelled Mars Sample Return in my opinion were adequate [billions of dollars is much more significant than very small chance of dangerous comtamination]. And/or if we going to Mars with a Mars crew to look for life, we shouldn't comtaminate the mars area we are exploring with Earth's microorganisms, because that could waste the billion dollars spent for Mars manned mission. Of course, I don't think the purpose of Manned Mars should be to find Martian life, the purpose I think should be to explore Mars for future colonization- and looking for Martian life would be a fairly low priority. If your holy grail is finding alien life, I think Mars Sample Return could one path, though I believe the chances are small, there could other aspects of mission which could have other benefits- you might find fossil evidence, or reasons why there can't be life. Other than this, checking out Europa probably has the biggest chance of success.]