We can't be so sure. The probes have discovered that Mars has no channels and vegetation. That water is uncommon (then discovered that it is still there in some quantity). They found out precise atmospheric composition, mapped out all major surface features, observed the climate over decades. They discovered perchlorate toxicity of the soil for humans, something that would have been a nasty surprise to a manned crew.

Am not opposed to Mars expeditions in principle, it's an exciting thought. But I just can't see humans contributing all that much on the odd few landings, with a high chance of contaminating whatever traces of life there could be.