I think its ridiculous to think we'd be like the Humans in Avatar. It makes no sense at all, and I believe we have, and are being visited, but see no reason aliens would invade. If you can travel through space to far off planets, why would you land in a partially farmed planet and kill its people to take their resources?
There would be infinite lifeless planets to choose from with resourses fresh for the picking. If we're being visited I think aliens are just checkin us out, seeing how we progress, maybe preventing our potential doom, keeping an eye on nukes, poppin asteroids on a collision course with earth.
I also think that is what we would do. We'd examine them, it would be fascinating. We wouldn't obliterate them, we can bend time and travel anywhere almost instantaneously. Why wipe out the poor little guys.