I've always felt that if there are any aliens actually buzzing around Earth in flying saucers, they are most likely biologists here on scientific
safaris, studying us the way Jane Goodall and others have spent their careers studying apes and other wildlife. Maybe they are (at best) their
equivalent of primatologists, studying what they consider to be semi-intelligent life forms, or maybe they are more like entomologists, studying what
are to them no better than insects ("look how clever they are building those vast habitat colonies for themselves, such industrious creatures...").
It's also just possible that we are, to them, like a stone age civilization persisting into the modern age, like some tribe deep in the Amazon, or on
some South Seas island and they are wary of ruining the natural development of our culture. Not unintelligent, just undeveloped. It's been the
experience here on Earth that when modern man makes contact with very primitive cultures, it tends to destroy those societies very quickly. Sometimes
it's the introduction of diseases, or a new technology throws everything in their society out of wack. The results can sometimes turn brutal.
So I agree, the invasion scenario doesn't make much sense, have long felt that for all the reasons you state. Once one understands just how much more
STUFF is out there, the same stuff in terms of resources, and there for the taking without a fight, it just makes no sense to bother going to an
inhabited world to get it. There are far more worlds out there that can't support life, never mind asteroids which are going to be much easier to
extract material from because of lower gravity. If you need food and you are a type-2 civilization you've probably long since figured out how to grow
the stuff hydroponically or even grow flesh in a manner similar to hydroponics, something that has already been invented by humans, though it's not
yet easy to do.
I think even using the Earth for living space because we happen to have a very similar environment is again super unlikely, but also probably silly
for a Type-2 civ as, again, they would by then have the technology and ability to terraform other worlds or even build large, sustainable artificial
dwellings in space along the order of a Halo-type ring. They might even be capable of building Ringworlds, like from the great novels by Larry Niven
of the same name or even the next step after that Dyson swarms and Dyson spheres. If living space is your problem, a ringworld sized construction is
going to solve those problems for a very, very long time to come. The one depicted in the books is millions of times larger than the Earth and a
Dyson sphere is an order of magnitude larger than that.
edit on 4/25/2012 by LifeInDeath because: (no reason given)