OK, I get it: alien invasion of Earth is a fascinating thought experiment. We’re all familiar with the history of exploration and colonialism, and it’s a useful exercise to imagine all of humanity on the wrong side of it -- preferably in the face of something mean and scaly.
But articles like this one (repeating to some extent Stephen Hawking’s recent musing on the subject) that talk about hordes of aliens skipping from planet to planet stripping them of resources miss one important point: Invading an occupied planet for its resources is stupid, stupid, stupid. Here’s the thing: there’s almost nothing on this planet that cannot be found, in abundance, on the various moons, asteroids, comets, and other small bodies in our solar system. Water? All over the place. Hydrocarbons? Maybe, but there are smaller moons that look pretty good for that, at least for the simple ones . Gold, iron, platinum, uranium, tritium: there for the taking.
What do those sources not have? Angry defenders (to be brushed away like gnats, natch) and a big honking gravity well (Link found quickly and easily via DCKX!). Yup, everything stolen from us pathetic Earthlings has to be hauled uphill at considerable cost. Whereas to drive off aliens stealing our asteroids, we would have to develop ships to go attack them. Even if you were a total badass, if you’re that interested in resources, you’ve probably got accountants who will tell you the right answer.
So, the moral of the story is, if aliens ever rain death upon us, it won’t be so they can steal our gold, it will be because they simply want to kill us. Possibly because they watched Glen Beck.
Now, there do remain potential invasion scenarios which do pop up from time to time. First possibility: capture a whole lot of semi-intelligent hominids to use as slaves, mining marshmallow peeps on Regulus V. (What? Where do YOU think they come from?)  Second possibility: Settlement, either permanent or temporary. (Both of those say a lot about the conquering aliens, in a “why do you have this technology, but not this other one?” way that leads to interesting books) Now, Prof. Hawking could argue that these are highly likely to be the case, but I’m not nearly so sure.
 This leads to the hilarious-to-me situation where “Aliens invade Earth for its oil” is actually the most plausible of the “resource stealing” scenarios... and is the one that nobody will touch because it’s just too corny and heavy-handed. Gotta love that.
 My personal theory is that Earth is a secret sweatshop for an alien race that really, really likes second-hand plastic.