OK, that is a bit of an over simplification of what Stephen said. In regards to abandoning Earth, he was merely talking about the statistical probability that at some point we will face an extinction event. The event may be man made or natural (nuclear war vs. volcanic activity or asteroid impact) but must at some point happen. If we are to survive as a species in a universe with an uninhabitable Earth we will have to survive somewhere else.
In regards to the alien contact situation, it makes perfect sense on two levels. On one, he spoke about microbes as opposed to intelligent life. Look what happens when organisms that exist on one portion of Earth move to another. It is often devastating for the people involved. As a point of reference look at what smallpox did to the Indians. Now, take a microbe that is not only a mutation of something already found on Earth (like drug-resistant bacteria) or new versions of known organisms (like HIV was a new form of virus) but a completely new alien species. How long do you think it would take a human immune system to learn to fight off not only a completely alien virus but a kind of microbe which we never saw?
On the other hand intelligent alien life, he reckons, will pose a completely different danger. He hypothesized that any alien species that spent the time and resources to seek out other life wouldn't do it for benign reasons. They would be seeking something like an energy source or natural resources and we would merely be obstacles in the way of extracting those things from our planet.
I don't pretend to understand everything Stephen talks about but it is a bit disingenuous to over simplify something he says and then criticize him based on that over simplification. Technically both he and Einstein believe time travel into the future (though not past) is possible if we can travel at the speed of light. That too sounds ridiculous if you don't understand what he is actually saying, that you can change how time passes for you in relation to others if you can approach the speed of light. I would hardly call Stephen knucklehead.
Dalton has an interesting view of things. All in good fun. 
