Practically, it wouldn't make a damn bit of difference.
Philosophically, I suspect it would rock a lot of peoples' worlds -- proof that we aren't alone in the universe, proof that we're not unique and special, proof that not only is life relatively common, it's a lot more common than we think, proof that investment in space exploration might have a lot more interesting payoff than we might otherwise think, etc. It wouldn't prove, but certainly call into question whether life even originated on Earth. It would lend credance to a lot of big speculation about big questions that would otherwise get dismissed as science fiction ... even if we're only talking about a few microbes.
Plus sales of Ray Bradbury's books might jump ...