Unfortunately it's also all but inevitable that we'll bring some with us, if we haven't already. Life is persistent, and ever-surprising.
It's hard to justify the required R&D expenses without appealing to emotion (such as sending humans).
I guess also wrapped into my statement was the thought that to be able to launch something back it would have to be landed very well and require some very complex machinery to prepare for launch, given the success rates of just being able to land something on Mars without it breaking. I went with the first reason off the top of my head :)
https://en.wikipedia.org/wiki/Sabatier_reaction#Manufacturin...
But there are Mars rocks that have been busted loose from Mars, and orbited down to Earth.
But I think it's safe to assume it's quite fragile at the very beginning, and therefore conditions must be milder for it to appear at all.
Once it takes off it's probably much more sturdy.
It may still be that life is so unlikely that it only happens on one out if every billion planets where the right conditions exist.
Yep. Here's a great book on the topic:
http://www.amazon.com/Travelers-Guide-Mars-William-Hartmann/...