Hawking's watched Avatar too many times

That's what I'd figured when I saw the headlines about Stephen Hawking's worries about aliens wanting to take our stuff. Unless Earth has particular elements in abundance that are rare elsewhere, wouldn't it be easier just to mine it out of unpopulated asteroids? Surely a species seeking to master interstellar travel would have worked out how to mine non-class-M planets.

Dan Drezner comments more thoroughly:

2)  Carried to its logical extreme, isn't Hawking making an argument for rapidly exhausting our natural resources?  If Hawking is correct, then the sooner we run out of whatever might be valuable to aliens, the less interest we are to them.  Of course, this does beg the question of which resources aliens would consider to be valuable.  If aliens crave either sea water or bulls**t, then the human race as we know it is seriously screwed. 

3) Why would aliens go after the inhabited planets?  Ceteris paribus, I'm assuming that aliens would prefer to strip-mine an uninhabited planet abundant with natural resources than an inhabited one.  Three hundred planets have already been discovered in the Milky Way, and there are "likely many billions."  Even rapacious aliens might try some of them first before looking at Earth, since we are mostly harmless. 

There is a counterargument, of course.  Over at Hit & Run, Tim Cavanaugh tries to assuage fears of aliens by observing, "Why would a race of superintelligent jellyfish or blue whales even take notice of us, let alone want to conquer us?"  This cuts both ways, however.  If those jellyfish fail to notice us but notice our abundant amounts of salinated water, they could decide to come without a care in the world for the bipedal inhabitants of Earth.