"You know I don't think I've ever heard a good reasoned arguement for planetary protection(lets not mention any names). It woudl be interesting to see them. I can't help but wonder maybe Zubrins views are just knee jerk reacitons to ICAMSR?"

I think the best argument is we simply don't know.
I think it's fairly unreasonable to expect some human disease from any life (if there is life) on Mars.
BUT:
The fact is that we aren't the dominant life form on Earth- microbes have survived longer, we depend on them for our existence, there is more biomass in microbial form then all the biomass of all "higher lifeforms" [including trees and ants]. We can't know very much about this life, since most of it has only been recently discovered to be miles underground.
What we don't know is HUGE- we commonly delude ourselves that we "almost" know everything. We do know alot, we got a fairly good grasp of the basics, but there is so much more. I think the proof that we don't "know everything" is the rate in which we are scientifically advancing.
It's also about "protecting the evidence". What's the value of spending billions of dollars if you get a "uncertain result". If we find life in the sample, we should have high confidence it's not some stupid earth contamination. Also we don't want to contaminate Mars, for same reason.
It also depends on why are we exploring Mars? What are NASA responsiblities as a governmental organization? Etc.