Furthermore, what if life on Mars were fragile enough that Earthly microscopic organisms destroyed it?
Besides, if there are primitive life forms on Mars, it's quite presumptuous of us to assume we could so easily wipe them out.
Mars becomes infinitely more interesting if it can become a host for terrestrial life.
It's not like we should care about the consciousness of microbes. What he meant, as I read it, is that if there are microbes we'll want to sample them and study them BEFORE we change the landscape and possible kill them.
I don't think we want to contaminate or destroy our only source of knowledge on other kind of life.
May be we can do that later, once we study them enough or have kept it enough for further study. But let's not make our first decision be "We gonna destroy it someday anyway, let's just stomp on it carelessly right now"
Finding a life outside of Earth will be one of the greatest discoveries of man kind, something that will never be overshadowed by anything. They want to make sure that when we'll find something it didn't catch a ride on what ever we sent to find it.
Eventually when we start exploring space it wouldn't really matter, yes some precautions might still be kept but we wouldn't take as much care as we do now.
I for one also do not see humans contaminating an alien ecosystem as something unnatural, humans evolved, they've spread out, they seed life (intentionally or otherwise) this is just as much of a natural process as the potential for comets and meteorites to seed solar systems and beyond with organic compounds and even life it self.
There would definitely be a difference vis-a-vis reconstructing the natural history of Mars.