According to Hawkings, any alien race advanced enough to reach Earth will likely not be friendly - chances are excellent they have already exhausted the resources of their own world and would only come here to rape our planet.
I disagree with that, but I do agree it would be a catastrophe to contact alien civilizations. First, if they can reach Earth, they can reach any place in the Universe. The universe is overflowing with resources, so they do not need the ones found here. But...
If they are so advanced as to be able to reach us, then we would be little more than a far inferior species, and they would likely view us as we view cattle. We would either be used as a food supply, or as slaves to help them gather resources and grow food for them. It would be much like the Egyptians and the Hebrews, only 100 times worse.
And lest we forget - we cannot even get along with other humans. Nations fight. Races fight. Ideologies fight. So what chance is there that we could get along with another race completely unlike anything we could ever imagine? No chance at all. There would be war. And their advanced superiority would assure that we would lose.
If you ask me, we should not be so anxious to contact other civilizations...their best-selling book may be "How To Serve The Human Race". And it just might be a cookbook.