The guy is going bonkers, making the true academic physics looks like a last week’s tabloid. He is moving on from theorizing the beginning of universe and fate of black holes to dos and donts of socializing with intelligent alien life form.
Well, he believes that presence of alien life form is reality. His belief is based on statistical probability. And this belief is shared by majority of scientists. So much so that NASA has sent messages in various parts of the universe saying something like "hum yahan hein, come and visit us".
Hawking is saying that attempts like this are dangerous, because if aliens do reach Earth then encounter is probably not going to be pleasant.
I think he makes sense. To me it is nothing science-fictional.
Just imagine what will humans do when they reach less-developed aliens. Actually we just have to look at the history. Every time one people have reached another place, the result has most been disastrous for indigenous population. Probably that's what Hawkings had in mind when he made this warning.
If we reach aliens before they reach us, we'd probably be technologically more advanced and vice versa.
Avatar is a case in point :P
For me, the alien life forms might actually be out of our perception boundaries, where our sensory limitations might lead us not to see, hear or feel them. Better to stay away then!