I'm with Tom
If we found life on Mars it'd raise a mountain of ethical questions about whether we have any right to muscle our way onto their turf, wouldn't it? Even if "they" were microbial. At least, I *hope* it'd raise such questions.
But if it's lifeless, then we could explore, and eventually develop, to our heart's content without despoiling an existing ecosphere. It'd certainly simplify any future efforts to make Mars part of our domain--again assuming we'd show respect for indigenous life if we found it. I think we would, though there's a depressing lack of historical precedent suggesting so.