So my pet peeve? SF writers who have aliens traveling light-years across the galaxy to steal our water. (By "our" water, I mean the Earth's water. I leave to the reader the exercise of determining whether humans, vis-a-vis aliens from another star, "own" the Moon's ice, or Saturn's rings).
Why take our water? It's at the bottom of a gravity well and surrounded by angry primates with nuclear weapons and a self-destructive streak.
There are things that invaders might want: heavy elements, which tend to congregate in rocky inner worlds, biological products that are unique to Earth's biosphere, etc. Or they may just want to eliminate potential future competitors (http://news.discovery.com/space/alien-life-exoplanets/space-invaders-unlikely-for-now-130214.htm).
Just don't use water as your crutch, writers. Pick something unique!